MB-8165: Allow bg fetches from replica vbuckets

This is needed in order for the replica read command to work
properly.

Change-Id: I45356d92776a24f1cabccdbf1ccff4fe0845b5c8
Reviewed-on: http://review.couchbase.org/26429
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>