rbd: define rbd snap context routines

Encapsulate the creation of a snapshot context for rbd in a new
function rbd_snap_context_create().  Define rbd wrappers for getting
and dropping references to them once they're created.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>