relay: fix segfault on replica transition from anonymous

relay_subscribe_f sets a recovery trigger notifying tx when a full log
is read and gc consumer corresponding to the replica may be advanced.
Since anonymous replicas do not have gc consumers, the trigger isn't
added for them. However, on relay exit, the trigger deletion depends
on replica->anon flag. This is buggy in case relay stalls on exit due to
replica disconnect. Replica has time to reconnect and register as a
normal instance, hence its replica->anon flag will be false by the time
we check whether to clear triggers or not, effectively making us to
clear unset triggers and segfault.

Fix this by initializing the triggers with trigger_create(), which
allows a trigger_clear() call, even if the triggers are not set, and
omit the replica->anon check.

Closes #4731

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>