Fix period based stream flushing. Adds flush-period and flush-threshold URL param support to the streaming endpoints.
Fixes stream flush and adds better stream flush logging.
Fixes test comment.
Adds flush to stream event to make it obviously a client flush and not a server flush.
Merge pull request #178 from Shopify/predupsort_stream_flush

Pre-dupsort: fix streaming endpoint flushing and add support for flush-period and flush-threshold URL parameters