librbd: simplified retrieving snapshots from image header

librbd: simplified retrieving snapshots from image header

Added a new cls 'snapshot_get' method that retrieves all snapshot
specific details to avoid the need to add a new cls method to
retrieve each individual snapshot data field.

The cls 'get_snapshot_namespace' method has been dropped since Luminous
clients will treat that as a user-generated snapshot (and that's all
that is supported by Luminous clients). The cls 'get_snapshot_timestamp'
method has been deprecated and can be removed once <=Luminous clients
are no longer supported.

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