rbd-mirror: fix potential deadlock in finish_shut_down()

rbd-mirror: fix potential deadlock in finish_shut_down()

Problem:
A race condition could lead to a deadlock in finish_shut_down(). The issue
occurs when the routine attempts to acquire m_lock while it is already held
by one of the Replayer callbacks, such as validate_local_group_snapshots() or
create_group_snapshot().

Solution:
Refactored finish_shut_down() to run asynchronously by splitting it into two
routines, wait_for_in_flight_ops() and handle_wait_for_in_flight_ops(). The
handle_wait_for_in_flight_ops() function acquires the lock, but now executes in
a separate thread, avoiding lock contention and eliminating the deadlock risk.

Credits to Ilya Dryomov <idryomov@gmail.com>

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Resolves: rhbz#2411963