Background jobs

This changeset introduces an abstraction for (maintenance) jobs running in the background, of which execution may be triggered from other threads. SubscriptionManager could very well use this, but I haven't made it do so yet.

@xor-freenet Unit tests are not complete, I'll be finishing those tomorrow. I'm filing this pull request early so you can have a look at it already.
- Having an adjective here ("oldest") sounds as if there were multiple triggers, but there aren't.
- If you rename the trigger() function as I suggested, you might as well rename this then.