Don't pass a callback to the enigne when invoking add_stream

We don't need this anymore since it should be taken care of by
calls to the step api

Change-Id: I8eec91de2a05016bd6a45fcb96834c7e19c5fb68
Reviewed-on: http://review.couchbase.org/30250
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>