Create SelfUpdatingSse to pass to HttpResponse.

* Big change from last commit...SSEView is no longer an iterator.
* Rather than make SSEView an iterator, create SelfUpdatingSse
  (subclass of Sse) as an iterator and pass it to HttpResponse.
* Add EventReader base class.
* Add CacheReader class (derived from EventReader.)
* EventReader object has a configurable timeout and sleep_interval.
* SelfUpdatingSse uses an EventReader object to read new events
  continually, yielding them as it reads them.
* SelfUpdatingSse has a close() method that is called at the end of
  the request and is used for closing DB or Redis connections.