Hold streamMutex whenever we are checking the itemsReady value

Change-Id: I609c6ad6803910979fc7f9aacc82a4e7640e411b
Reviewed-on: http://review.couchbase.org/35119
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>