paxos_state: prepare: handle exception getting data or digest
paxos_state: prepare: handle exception getting data or digest

This exception is ignored by design, but if it's left unhandled,
it generates `Exceptional future ignored` warnings, like the following.

Also, ignore f2 if f1 failed since we return early in this case.

```
[shard 5] seastar - Exceptional future ignored: seastar::named_semaphore_timed_out (Semaphore timed out: _read_concurrency_sem), backtrace: 0x431689e 0x4316d40 0x43170e8 0x3f35486 0x218d14a 0x3f8002f 0x3f81217 0x3f9f868 0x3f4b76a /opt/scylladb/libreloc/libpthread.so.0+0x93f8 /opt/scylladb/libreloc/libc.so.6+0x101902#012
N7seastar12continuationINS_8internal22promise_base_with_typeISt7variantIJN5utils4UUIDEN7service5paxos7promiseEEEEEZZZZNS7_11paxos_state7prepareEN7tracing15trace_state_ptrENS_13lw_shared_ptrIK6schemaEERKN5query12read_commandERK13partition_keyS5_bNSI_16digest_algorithmENSt6chrono10time_pointINS_12lowres_clockENSQ_8durationIlSt5ratioILl1ELl1000EEEEEEENK3$_0clEvENUlvE_clEvENKUlSB_E_clESB_EUlT_E_ZNS_6futureISt5tupleIJNS13_IvEENS13_IS14_IJNSE_INSI_6resultEEE17cache_temperatureEEEEEEE14then_impl_nrvoIS12_NS13_IS9_EEEET0_OS11_EUlOSA_RS12_ONS_12future_stateIS1B_EEE_S1B_EE#012
seastar::continuation<seastar::internal::promise_base_with_type<std::variant<utils::UUID, service::paxos::promise> >, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::finally_body<seastar::with_semaphore<seastar::semaphore_default_exception_factory, seastar::lowres_clock, service::paxos::paxos_state::prepare(tracing::trace_state_ptr, seastar::lw_shared_ptr<schema const>, query::read_command const&, partition_key const&, utils::UUID, bool, query::digest_algorithm, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >)::$_0::operator()() const::{lambda()#1}>(seastar::basic_semaphore<seastar::semaphore_default_exception_factory, seastar::lowres_clock>&, unsigned long, seastar::lowres_clock::duration, std::result_of&&)::{lambda(seastar::basic_semaphore)#1}::operator()<seastar::semaphore_units<seastar::semaphore_default_exception_factory, seastar::lowres_clock> >(seastar::basic_semaphore)::{lambda()#1}, false>, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::then_wrapped_nrvo<seastar::future<std::variant<utils::UUID, service::paxos::promise> >, seastar::semaphore_units<seastar::semaphore_default_exception_factory, seastar::lowres_clock> >(seastar::future<std::variant<utils::UUID, service::paxos::promise> >&&)::{lambda(seastar::internal::promise_base_with_type<std::variant<utils::UUID, service::paxos::promise> >&&, seastar::semaphore_units<seastar::semaphore_default_exception_factory, seastar::lowres_clock>&, seastar::future_state<std::variant<utils::UUID, service::paxos::promise> >&&)#1}, std::variant<utils::UUID, service::paxos::promise> >#012
seastar::continuation<seastar::internal::promise_base_with_type<std::variant<utils::UUID, service::paxos::promise> >, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::finally_body<service::paxos::paxos_state::key_lock_map::with_locked_key<service::paxos::paxos_state::prepare(tracing::trace_state_ptr, seastar::lw_shared_ptr<schema const>, query::read_command const&, partition_key const&, utils::UUID, bool, query::digest_algorithm, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >)::$_0::operator()() const::{lambda()#1}>(dht::token const&, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >, std::result_of)::{lambda()#1}, false>, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::then_wrapped_nrvo<seastar::future<std::variant<utils::UUID, service::paxos::promise> >, {lambda()#1}>({lambda()#1}&&)::{lambda(seastar::internal::promise_base_with_type<std::variant<utils::UUID, service::paxos::promise> >&&, {lambda()#1}&, seastar::future_state<std::variant<utils::UUID, service::paxos::promise> >&&)#1}, std::variant<utils::UUID, service::paxos::promise> >#012
seastar::continuation<seastar::internal::promise_base_with_type<std::variant<utils::UUID, service::paxos::promise> >, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::finally_body<service::paxos::paxos_state::prepare(tracing::trace_state_ptr, seastar::lw_shared_ptr<schema const>, query::read_command const&, partition_key const&, utils::UUID, bool, query::digest_algorithm, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >)::$_0::operator()() const::{lambda()#2}, false>, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::then_wrapped_nrvo<seastar::future<std::variant<utils::UUID, service::paxos::promise> >, service::paxos::paxos_state::prepare(tracing::trace_state_ptr, seastar::lw_shared_ptr<schema const>, query::read_command const&, partition_key const&, utils::UUID, bool, query::digest_algorithm, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >)::$_0::operator()() const::{lambda()#2}>(service::paxos::paxos_state::prepare(tracing::trace_state_ptr, seastar::lw_shared_ptr<schema const>, query::read_command const&, partition_key const&, utils::UUID, bool, query::digest_algorithm, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >)::$_0::operator()() const::{lambda()#2}&&)::{lambda(seastar::internal::promise_base_with_type<std::variant<utils::UUID, service::paxos::promise> >&&, service::paxos::paxos_state::prepare(tracing::trace_state_ptr, seastar::lw_shared_ptr<schema const>, query::read_command const&, partition_key const&, utils::UUID, bool, query::digest_algorithm, std::chrono::time_point<seastar::lowres_clock, std::chrono::duration<long, std::ratio<1l, 1000l> > >)::$_0::operator()() const::{lambda()#2}&, seastar::future_state<std::variant<utils::UUID, service::paxos::promise> >&&)#1}, std::variant<utils::UUID, service::paxos::promise> >#012
seastar::continuation<seastar::internal::promise_base_with_type<seastar::foreign_ptr<std::unique_ptr<std::variant<utils::UUID, service::paxos::promise>, std::default_delete<std::variant<utils::UUID, service::paxos::promise> > > > >, service::storage_proxy::init_messaging_service()::$_51::operator()(seastar::rpc::client_info const&, seastar::rpc::opt_time_point, query::read_command, partition_key, utils::UUID, bool, query::digest_algorithm, std::optional<tracing::trace_info>) const::{lambda(seastar::lw_shared_ptr<schema const>)#1}::operator()(seastar::lw_shared_ptr<schema const>)::{lambda()#1}::operator()() const::{lambda(std::variant<utils::UUID, service::paxos::promise>)#1}, seastar::future<std::variant<utils::UUID, service::paxos::promise> >::then_impl_nrvo<{lambda()#1}, {lambda()#1}<seastar::foreign_ptr<std::unique_ptr<std::variant<utils::UUID, service::paxos::promise>, std::default_delete<std::variant<utils::UUID, service::paxos::promise> > > > > >({lambda()#1}&&)::{lambda(seastar::internal::promise_base_with_type<seastar::foreign_ptr<std::unique_ptr<std::variant<utils::UUID, service::paxos::promise>, std::default_delete<std::variant<utils::UUID, service::paxos::promise> > > > >&&, {lambda()#1}&, seastar::future_state<std::variant<utils::UUID, service::paxos::promise> >&&)#1}, std::variant<utils::UUID, service::paxos::promise> >#012
seastar::continuation<seastar::internal::promise_base_with_type<seastar::foreign_ptr<std::unique_ptr<std::variant<utils::UUID, service::paxos::promise>, std::default_delete<std::variant<utils::UUID, service::paxos::promise> > > > >, seastar::future<seastar::foreign_ptr<std::unique_ptr<std::variant<utils::UUID, service::paxos::promise>, std::default_delete<std::variant<utils::UUID, service::paxos::promise> > > > >::finally_body<seastar::smp::submit_to<service::storage_proxy::init_messaging_service()::$_51::operator()(seastar::rpc::client_info const&, seastar::rpc::opt_time_point, query::read_command, partition_key, utils::UUID, bool, query::digest_algorithm, std::optional<tracing::trace_info>) const::{lambda(seastar::lw_shared_ptr<schema const>)#1}::operator()(seastar::lw_shared_ptr<schema const>)::{lambda()#1}>(unsigned int, se
```

Refs #7779
Refs #9331

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20210919053007.13960-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla