Deployer

Adds a deployer component, which uses the queue (#547) to schedule deployments using different strategies (currently one-by-one or all-at-once).

Resolves #602.
The queue package will [unlock jobs of dead workers](https://github.com/flynn/flynn/blob/queue/pkg/queue/queue.go#L38) so we shouldn't re-queue these jobs.

I think we need to handle this using a lease as described in #603.