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.
How do we know there won't be any collisions? i.e., two different feeds that result in the same name?