Database cleanup job

[jplitza]

The IDs of handled events accumulate over time, so they need to be cleaned up! (For reference, my database is still <6MB after 2 years of operation.)

We probably need to introduce a timestamp column in the events table that gets updated every time we see the ID in a feed. Then we could delete IDs that haven't been seen for a configurable amount of time (like 7 days).