MB-11299: Allow rollbacks to a partial snapshot

If we roll back to a partial snapshot we need to make sure to get
the snapshot start and end seqnos for the partial snapshot in order
to make sure that we don't accidentally miss any items when
reconnecting.

Change-Id: If95d0476d8d582d6b57786387212f03b2aafc7ba
Reviewed-on: http://review.couchbase.org/40340
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>