Close requests to /sse after 30 sec. timeout.

* Use gevent.Timeout().
* Remove sse_keepalive() task.
* Remove Celery beat, since the "keepalive ping" was
  the only use for periodic tasks.
Fix termination of EC2 instance.
Remove Trunk module.

* No longer using Postgres notifications.
Add settings for memcache.
Add SseView to replace PostgresQueueView.

* Cache last_updated timestamp in cache.
* SseView repeatedly polls cache for last_updated value
  and triggers page refresh on change.
* Previous PostgresQueueView worked, but required many
  connections to DB, which are limited on Heroku dev plan.
Call send_event() for every instance state change.

* Also, make the "now" timestamp the default for send_event().
Add Memcachier add-on instructions to README.
Add runtime.txt to specify Python version.
Cache last_updated value for a year.
Simplify cache use in sseview.py.

* Cache instance_state value, not time last updated.
* Now, use infinite loop in SSE iterator. No longer use
  gevent.Timeout, since Firefox (on Mac) was not retrying
  /sse after timeout.
Make SSEView an iterator.

* Use __iter__() and next() so that I can raise
  StopIteration when needed.
* Previously, when clients disconnected, the infinite loop
  in iterator() caused a memory leak, so it's better to use
  next() so I can return a message at a time.
* Track running time, so I can raise StopIteration
  when timeout is reached.