Use task names to reduce the risk of duplicate update-feed tasks.

The task name is based on the feed id (URL). Setting this should reduce the risk of duplicate update-feed tasks (#231) because the queue shouldn't accept tasks with duplicate names. According to the docs this isn't an iron guarantee, but should work under most circumstances.

I've been running this change self-hosted for a day or so. I'm not sure whether or not it's caught any duplicates, but it doesn't seem to cause any new problems.
Task names look like `2014-12-16T16-58-34Z07-00_http_3A_2F_2Fxkcd_2Ecom_2Fatom_2Exml`: the time to 1-sec precision plus the full URL in escaped form. I think 1-sec is plenty precise enough to avoid collisions, but we could add fractional seconds if needed.

With that format I don't see how we could generate collisions — so I must be missing something. Can you explain?