MB-17889: No notify from streamRequest

MB-17889: No notify from streamRequest

The 3.1.3 (i.e. before the DCP churn) didn't ever notify from
streamRequest, so that is reverted and helps to bring
view_query latency down (view engine is constantly creating
streams).

A second tweak is to not call stream->next whilst holding
the streamMutex. This can block streamRequest again
affecting the view-engine's DCP stream requests.

Change-Id: I5b57fd7998003251fb32897f37c8a2f15f687a13
Reviewed-on: http://review.couchbase.org/60352
Reviewed-by: Manu Dhundi <manu@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>