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 would like to compliment you for grouping member variables by semantic relationship by having this between the above constant-over-lifetime ones and the below non-constant ones - such attention to detail does not go unnoticed! :)