Implement remaining change feed response types

We need to figure out how to best implement the longpoll, continuous and eventsource types for the change feed. See http://docs.couchdb.org/en/latest/api/database/changes.html#changes-feeds

The longpoll type might not be too difficult, we just need a while-loop that polls the sequence index and exits and returns a change when there's a new sequence entry.

The continuous type will be more difficult. We need to find out a way to do streaming responses in Drupal. This might give us a start: http://stackoverflow.com/questions/12380783/streaming-a-response-in-symf...