librbd: avoid blocking AIO API methods
Enqueue all AIO API methods within the new librbd thread pool to
reduce the possibility of any blocking operations. To maintain
backwards compatibility with the legacy return codes of the API's
AIO methods, it's still possible to block attempting to acquire
the snap_lock.
Fixes: #11056
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit
3a7b5e30efdb21aa1a0aeb68a5d02a1ac2a5faf3)
(cherry picked from commit
cf6e1f50ea7b5c2fd6298be77c06ed4765d66611)