Add extra upr stats for last seqno sent and items ready

This change adds two stats that displays the last sequence number
sent as well as whether a stream has items ready to be sent.

Change-Id: I01a6b239e675d12a0e4354931c81d43a37f03fa3
Reviewed-on: http://review.couchbase.org/34901
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>