rbd-mirror: fixes multiple issues in the group replayer

rbd-mirror: fixes multiple issues in the group replayer

The commit includes the following:
- Fixed crashes in the start/stop in GroupReplayer
- Fixed crashes in the shut_down sequence in group_replayer::Replayer
- ImageMap will now send release_group notifications for non-empty
  groups.
- InstanceReplayer no longer checks if the GroupReplayer needs to be
  restarted. The GroupReplayer will stop itself if it determines that it
  needs to be restarted.

Signed-off-by: N Balachandran <nithya.balachandran@ibm.com>