librbd: break composite cls rbd helpers into individual helpers

librbd: break composite cls rbd helpers into individual helpers

With the forthcoming need to use new methods to retrieve a clone's parent,
use per-method helpers to invoke the correct method within the image
refresh state machine.

Also used this time to merge newer cls rbd methods that should be supported
by all non-EOLed OSDs.

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