MB-11297: Ensure lastSentSeqno < endSeqno

As we send the complete snap shot, it can so
happen that lastSentSeqno be greater than endSeqno
for an active stream, causing an underflow in
stats collection.
Change-Id: I8a8f0755f988c8c1ef9046020a4702775d2ddd4c
Reviewed-on: http://review.couchbase.org/38353
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>