librbd: Create image exclusive lock watch/notify handler

The new watch/notify handler replaces the existing header
update watch/notify handler and add support for managing
image exclusive lock leadership.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
librbd: Integrate librbd with new exclusive lock feature

Operations that update the image now require the exclusive lock
if the feature is enabled.  AIO write and discard operations will
automatically request the exclusive lock from the current leader
to support live-migration.

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>
librbd: Add ImageWatcher unit test cases

Directly unit test the new ImageWatcher class to complement
the existing librbd integration tests of exclusive lock
handling.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
librbd: Add internal unit test cases

The new unit tests cover the modifications made to integrate
the internal librbd functionality with the new ImageWatcher.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
librbd: differentiate between R/O vs R/W RBD features

The new RBD exclusive lock feature should be treated as a
feature that is only applied when the image is opened in
R/W mode.

Older clients will need to handle the updated
cls_rbd::get_features method in order to properly determine
the incompatible features for an image depending on the
current mode.

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