service: migration_manager: retry schema change command on transient failures
service: migration_manager: retry schema change command on transient failures

The call to `raft::server::add_entry` in `announce_with_raft` may fail
e.g. due to a leader change happening when we try to commit the entry.
In cases like this it makes sense to retry the command so we don't
prematurely report an error to the client.

This may result in double application of the command. Fortunately, the schema
change command is idempotent thanks to the group 0 state ID mechanism
(originally used to prevent conflicting concurrent changes from happening).
Indeed, once a command passes the state ID check, it changes the group 0
history last state ID, causing all later applications of that same
command to fail the check. Similarly, once a command fails the state ID
check, it means that the last state ID is different than the one
observed when the command was being constructed, so all further
applications of the command will also fail the check (it is not possible
for the last state ID to change from X to Y then back to X).

Note that this reasoning only works for commands with `prev_state_id`
engaged, such as the ones which we're using in
`migration_manager::announce_with_raft`. It would not work with
"unconditional commands" where `prev_state_id` is `nullopt` - for those
commands no state ID check is performed. It could still be safe to retry
those commands if they are idempotent for a different reason.

(Note: actually, our schema commands are already idempotent even without
the state ID check, because they simply apply a set of mutations, and
applying the same mutations twice is the same as applying them once.)
Message-Id: <9a018c3279def3074444521bfadf81f385a2cdcf@scylladb.com>

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