Run many tasks more often

The release team wanted us to run the Britney task more often to reduce the delay until new data is visible on tracker.debian.org.

Doing this is currently not easily possible because we don't have any sane locking mechanism to avoid collisions between multiple runs of the same task and everything is closely tied to update events sent by the main update_repositories task that we don't really want to run more often right now.

So basically this has to wait until #8 (closed) has progressed enough.