tarantool / tarantool / 5287365697

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replication: fix waiting for remote ballot updates during bootstrap

There was a problem in applier_wait_bootstrap_leader_uuid_is_set():
 * it didn't set an error cause when the ballot watcher was dead,
   leading to a segfault when trying to diag_add() to the cause.
 * it didn't expect the ballot watcher to exit without an error
   before the bootstrap leader uuid is known, hanging forever.

The first issue could be reproduced by trying to bootstrap a replicaset
consisting of both "old" (Tarantool 2.10 or less) and "new" instances.
Or by bootstrapping a replicaset consisting of "new" instances and
stopping some of them in a specific manner.

The second issue could be reproduced only by manually broadcasting an
empty "internal.ballot" event.

Fix both issues. The first one by setting an ER_UNKNOWN error when
the ballot watcher fiber is dead. And the second one by checking if the
ballot watcher has died during waiting for the ballot update.

The test only covers the first issue: the second one can only happen
with manual intervention and is hard to test: it involves broadcasting
an empty "internal.ballot" from the replica while it's still connecting
to remote nodes during an initial `box.cfg{}` call.

Closes #8757

NO_DOC=bugfix

(cherry picked from commit 1c25efb4b)

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