Enable blocking worker resubmission

[mzouink]

When i submit a task with a bug on it, daisy try to solve it by submitting ~infinite number of workers.
Is it possible to set worker resubmission as option?

[pattonw]

I took a quick stab at this and it turns out it is surprisingly difficult to stop the worker pools since it seems there is quite a bit of code trying to make sure we restart workers on error, trying to be robust to unexpected failures.
For now I have pushed a SerialServer here that you could use for easier debugging of process_block functions.