agent: periodically re-publish active tasks

[psFried]

Controllers should periodically re-publish all active specs. We'd like to do this for several reasons:

• It limits the staleness of built specs, ensuring that all built specs in active use were produced by a relatively recent version of the agent.
• It allows us to limit the size of the builds storage bucket by setting a lifecycle policy

The actual interval at which we re-publish should probably be configurable via an application argument. If nothing else, it makes it easier to test. The default interval is up for debate. I'll start the bidding at 30 days.