librbd: initial config watcher implementation

librbd: initial config watcher implementation

The config watcher will initially observe all "rbd_" configuration
updates received from the MON that have not been locally overridden
at the pool and/or image level.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 54819a1aa95fe1be8b8ef44f8417e3fe82d9ec89)

Conflicts:
	src/test/librbd/mock/MockImageState.h
- once again, I don't see why git flagged this as a conflict

(cherry picked from commit c830365a8a243bd4976a682af1bf8d768924eaff)

Conflicts:
	src/librbd/ConfigWatcher.cc: image_lock -> md_lock
	src/test/librbd/mock/MockImageState.h: trivial resolution