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.
I don't think @code will produce a clickable link, it is merely about the font IIRC.
Use {@link JobState#IDLE} instead.