MB-11958: Make sure mutations and snapshot ranges are updated together

There is a race condition where an items seqno is updated and the
flusher runs and persists both the seqno and old snapshot range
before the new snapshot range can be updated.

Change-Id: If124e5ab05c45c68a6956d28cc3898f8c18df6a9
Reviewed-on: http://review.couchbase.org/40642
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>