rbd-mirror: make mirror properly detect pool replayer needs restart

rbd-mirror: make mirror properly detect pool replayer needs restart

When a PoolReplayer detects remote pool metadata change it
sets "stopping" flag expecting the Mirror will restart it.

Although setting "stopping" flag makes the PoolReplayer::run
thread to terminate, the thread's is_started function will still
return true until join is called (and reset the thread id).

This made impossible for the Mirror to detect (by calling
PoolReplayer::is_running) that the PoolReplayer needed restart.

Fixes: https://tracker.ceph.com/issues/54258
Signed-off-by: Mykola Golub <mgolub@suse.com>
(cherry picked from commit ad4a2990b87834fe4ae8c9111547d071aa6e75e5)