MB-11941: Update current vbucket snapshot when receiving tap items

We need to update the vbucket snapshot in order to support upgrades
because we initially have a tap stream and then move to a dcp stream.
Since tap doesn't support snapshots we can just set the current snapshot
to be the same as the seqno for that item.

Change-Id: I9da32df9366508276619507cf6cb9a66557e0919
Reviewed-on: http://review.couchbase.org/40582
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>