librbd: Create async versions of long-running maintenance operations

Resize and flatten now have async versions. The existing resize
and flatten operations now use the async versions internally. The
async operations will be used by the client holding the exclusive
lock when it receives maintenance requests from other clients.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
librados: Expose RadosClient instance id through librados

Allow librados clients to utilize the global id of the RadosClient.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
librbd: Add maintenance operation requests to ImageWatcher

Snapshot, flatten, and resize maintenance operations can now
be coordinated via the client holding the exclusive lock for the
image.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
librbd: Coordinate maintenance through exclusive lock leader

When the exclusive lock feature is enabled, only a single client can
modify the image.  As a result, certain maintenance activities
need to be proxied from the maintenance client to the active
leader.

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