Allow running workers in sub-processes

[EugeneKostrikov]

Currently all workers started by monq are running within single process. This puts certain limitations on the level on control monq has over the workers. For instance there's no way to interrupt a job as it runs, it's hard to utilise multi-core hardware while keeping the order of job etc.

A possible solution for it is to run separate workers as sub-processes. This way they can be esily killed completely interrupting job execution, have less impact on each other, monitored better etc.

To limit the scope for this issue we should only implement workers as sub-processes here.