Move changefeed listeners to web service

[heyheyjp]

Currently, the workers set up listening to db changefeeds and, effectively, queue items for their own work queues. This is problematic because if changes to the database occur that should be handled by a worker while the worker is not running, that change event is lost and the job is never processed. Instead of connecting the changefeed listeners in the worker process, we should do it in the standing web service. This also makes it easier to choose to alternative mechanisms for background task execution (instead of having always-running worker dynos).

[jeffreywescott]

For what it's worth, these used to be in the web service, but it caused performance problems during "heavy" load (i.e., voting), since JavaScript is single-threaded.

[heyheyjp]

Ah. Good to know. We might find this interesting in thinking about how to attack the issue:
https://www.rethinkdb.com/docs/sharding-and-replication/#running-a-proxy-node