MB-11980: Fix bugs caused from persisting the current snapshot

There are two issues fixed here. One is around the current
snapshot not being updated properly during bucket flush. The
second is an issue caused by persisting the wrong information
for the high seqno, snap start seqno, and snap end seqno during
the snapshot vbuckets task. These values should remain the same
as they were after the last vbucket persistence task otherwise
we might persist incorrect values.

Change-Id: Ie4a357270b52bd35b5f29018f24fd3550913f1cc
Reviewed-on: http://review.couchbase.org/40707
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>