Merge pull request #12364 from dillaman/wip-rbd-mirror-notifications

rbd-mirror: replace remote pool polling with add/remove notifications

Reviewed-by: Mykola Golub <mgolub@mirantis.com>

diff --cc src/test/rbd_mirror/test_ImageReplayer.cc
Simple merge

diff --cc src/test/rbd_mirror/test_PoolWatcher.cc
Simple merge

diff --cc src/test/rbd_mirror/test_fixture.cc
Simple merge