MB-11572 Use thread-safe RCPtr for stream instances.

Note that SingleThreadedRCPtr is not thread-safe if it is
accessed by more than one thread.

Change-Id: I561812b9cd70b9610fae7329b25baab0c582b9db
Reviewed-on: http://review.couchbase.org/38992
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>