rbd: Coordinate exclusive lock leadership via watch/notify

Expanded the existing image header update watch/notify mechanism
to support basic exclusive lock leadership actions.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
rbd: Image is now protected by an exclusive lock

When the RBD exclusive lock feature is enabled, all operations
that update the image can only be handled by the client which
holds the exclusive lock.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
rbd: Allow CLI to optionally create shared images

Images that are flagged as shared cannot use the RBD
object map nor RBD mirroring features.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>