MB-12063: Lock the current snapshot mutex when flushing items

We need to lock the current snapshot mutex before grabbing items
because it is possible that an itme might be queued, but the
current snapshot might be updated.

Change-Id: Ieff85a4605041e764d6facf055c71c59e7e19955
Reviewed-on: http://review.couchbase.org/40996
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>