JCBC-340: TAP backfill ignores replicas.

This changeset makes sure that only keys from the active
node get propagated to the TapStream. This helps, but is
not a full fix of the issue. See the corresponding linked
issues from JCBC-340 for the ongoing other fixes.

Change-Id: I4a7acaaa3066180be0e0949023c295aef2eb7685
Reviewed-on: http://review.couchbase.org/27916
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>