This adds abstract classes to base background jobs on.
Right now almost all uses of this use the private namespace.
For most usages it will be enough to just extend the the abstract
classes QueuedJob or TimedJob.
It should be a straight forward drop in replacement.
The private jobs can then be killed off after a few releases. So we have
a nice public API.
Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>
