librbd: attempt to recover lost image watcher upon all failures

librbd: attempt to recover lost image watcher upon all failures

For example, if an image is blacklisted and the blacklist eventually
expires, the image should recover its watch.

Fixes: http://tracker.ceph.com/issues/34534
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 23b7447f6be87a14f84664f29431d2fdd2af4512)

Conflicts:
	src/librbd/watcher/RewatchRequest.cc: trivial resolution
	src/test/librbd/CMakeLists.txt: trivial resolution
	src/test/librbd/test_mock_Watcher.cc: trivial resolution