MB-11299: Don't let replica vbuckets send partial snapshots

Replica vbuckets cannot create their own snapshots and must
rely on the snapshots it received from the active vbucket
when forwarding the snapshots to a downstream consumer. In
order to make sure that this happens properly we will only
stream mutations from the closed checkpoint for non-active
vbuckets. This will guarantee that the above requirement is
met.

Change-Id: I58151ce96868e34083f87dca0ce1d18b6a12aedf
Reviewed-on: http://review.couchbase.org/40346
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>