storage_service: Yield in get_changed_ranges_for_leaving
storage_service: Yield in get_changed_ranges_for_leaving

It is always called inside a seastar thread. Call yield to prevent
stalls.

This patch fixes stalls like:

   2019-12-17T15:18:33+00:00 ip-10-0-116-62 !INFO | scylla: Reactor stalled
   for 4609 ms on shard 0.

   0x0000000002accbd2
   0x0000000002a4579b
   0x0000000002a45cc2
   0x0000000002a45ff7
   0x00007ff0a609be7f
   0x0000000001b0b500
   0x0000000001b03185
   0x0000000001af0d41
   0x0000000001af027a
   0x0000000001f7e89a
   0x0000000001f9f55a
   0x0000000001fc9c09
   0x0000000001fcac08
   0x00000000007dfee3

   /jenkins/slave/workspace/scylla-3.2/build/scylla/seastar/src/core/reactor.cc:1041
   (inlined by) seastar::reactor::block_notifier(int) at
   /jenkins/slave/workspace/scylla-3.2/build/scylla/seastar/src/core/reactor.cc:1164
   ?? ??:0 __gnu_cxx::__normal_iterator<dht::token const*,
   std::vector<dht::token, std::allocator<dht::token> > >
   std::__lower_bound<__gnu_cxx::__normal_iterator<dht::token const*,
   std::vector<dht::token, std::allocator<dht::token> > >, dht::token,
   __gnu_cxx::__ops::_Iter_less_val>(__gnu_cxx::__normal_iterator<dht::token
   const*, std::vector<dht::token, std::allocator<dht::token> > >,
   __gnu_cxx::__normal_iterator<dht::token const*, std::vector<dht::token,
   std::allocator<dht::token> > >, dht::token const&,
   __gnu_cxx::__ops::_Iter_less_val) at crtstuff.c:?
   locator::token_metadata::first_token_index(dht::token const&) const at
   crtstuff.c:? locator::token_metadata::ring_range(dht::token const&, bool) const at crtstuff.c:?
   locator::simple_strategy::calculate_natural_endpoints(dht::token const&,
   locator::token_metadata&) const at crtstuff.c:?
   service::storage_service::get_changed_ranges_for_leaving(seastar::basic_sstring<char,
   unsigned int, 15u, true>, gms::inet_address) at crtstuff.c:?
   service::storage_service::unbootstrap() at crtstuff.c:?
   service::storage_service::decommission()::{lambda(service::storage_service&)#1}::operator()(service::storage_service&)
   const::{lambda()#1}::operator()() const [clone .isra.0] at
   storage_service.cc:?

Refs: #5495

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