Fix delete snapshots criteria

Fix delete snapshots criteria

When a snapshot is saved, we attempt to delete prior snapshots as only
the last one is recovered from persistence. For the maxSequenceNr in the
criteria, we use snapshot sequenceNr - snapshot batch count. However
this assumes every snapshot is based on snapshot batch count. We may
snapshot for other reasons, eg install snapshot on follower. This can
leave multiple prior snapshots behind and use up significant disk space.

We should only retain the last snapshot saved so I changed the criteria
to use: maxSequenceNr = the saved snapshot sequenceNr (it's possible the
sequenceNr hasn't changed from th elast snapshot) and maxTimeStamp =
saved snapshot timestamp -1.

Change-Id: I35b1d71ed433d52ecff79ca07a81616e393a7b7f
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>