rbd-mirror: leader watcher should not cancel get locker if locker is invalid

rbd-mirror: leader watcher should not cancel get locker if locker is invalid

When a new leader acquires the lock, it will send out a lock acquired
notification along with periodic heartbeats. The get locker will attempt to
run immediately, but if a heartbeat arrives before it executes the heartbeat
will cancel the timer and reschedule it for the future. This process repeats
for each periodic heartbeat and the locker is never re-read from the OSD.

This is an issue only for namespace replayers due to the delayed fashion in
which the leader instance id is retrieved.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>