MB-8903: Stream Request OK response returns Failover Log

couch_upr:enum_docs_since() now also returns the
Failover Log.

Change-Id: I2b940b8c62a31f741bcbfab7604c897b9859f345
Reviewed-on: http://review.couchbase.org/30836
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>