MB-11407: Update last_seqno in consumer after rollback

After a rollback is issued to an UPR consumer the last_seqno
needs to be updated to the rollback seqno. Otherwise, mutations
that are sent from the UPR producer could get dropped.

Change-Id: I910850be130e08b4a052e7d152736d625cee8765
Reviewed-on: http://review.couchbase.org/38823
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>