Add an ActiveStream class

The ActiveStream class will maintain the state for a stream that
is on the producer side of the connection. I wil add a similar
PassiveStream for consumer-side stream state in the future.

Change-Id: If884c335131da9f9138e63a2dda15d28fdc9a3b5
Reviewed-on: http://review.couchbase.org/31459
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>