Delete orphaned builds in batches

[kpcyrd]

There's a background job for cleanup that's likely looking up the database for extended periods of time:

https://github.com/kpcyrd/rebuilderd/blob/8ff458ff71381db01e1fba8abe01e974c7eacbf4/daemon/src/lib.rs#L36

This could be split up so the select is executed on its own, and the deletes are executed in batches, so each delete locks up the database briefly instead of locking it once for a long time.