relay: fix unterminated final_join thread

Currently if tarantool exits during relay's final join stage,
corresponding thread isn't terminated. This causes the flakiness
of the replicaset_ro_mostly.test.lua.

Let's reuse the same relay, in which subscribe cord is running, for
the final join stage. This way the cord will be cancelled during
replication_free().

Closes #8082

NO_DOC=not user-visible
NO_TEST=fix flaky test
NO_CHANGELOG=not user-visible

Co-authored-by: Sergey Petrenko <sergepetrenko@tarantool.org>