rbd-mirror: fix group replayer deleting the previous mirror group snapshot

rbd-mirror: fix group replayer deleting the previous mirror group snapshot

The prune logic in the group replayer currently holds the previous
group snapshot until the current group snapshot syncing is complete.
Hence there is always a previous group snapshot (and the respective
image snapshots).

But when there are user group snapshots group replayer takes them to
account and prunes the previous mirror snapshots, which shouldn't be done.
The group replayer should always hold the previous mirror group snapshot
(irrespective of the user group snap).

Also rename unlink_group_snapshots() -> prune_group_snapshots() and
substitute the use of unlink with prune as required.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>