Spike: job manager: breeejs out, node-cron in

[cmraible]

The job manager has one fatal flaw: it's not possible to schedule a recurring job that runs in the main thread; only in a worker thread.

In Ghost we've hacked around this by scheduling a worker job, then emitting an event back to the main thread to trigger a job to execute. This works, but it's essentially a really convoluted and expensive setTimeout()...why not just use a setTimeout()?

This spike rips out breejs entirely, and uses node-cron to handle the scheduling bits. This way we can just pass a function to addJob() and it will run on the provided schedule in the main thread, without having to spawn a worker.

This does mean we no longer support running jobs in worker threads at all, which we'll probably need to solve. There's nothing stopping us from launching a worker within the function we pass to node-cron, or even pass it to a pool of workers to handle.