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.
Also, as the constructor is not private, you might want to add a "@see DelayedBackgroundJobFactory Use DelayedBackgroundJobFactory to construct objects of this class, not this constructor."