Implement start and stop lifecycle hooks for the supervisor and workers

[rosa]

This is a simple first implementation for those. With this, you can register blocks of code to be called after the supervisor finishes booting but right before it forks workers and dispatchers (start) and before it starts to shut down, gracefully or immediately (stop).

It works like this:

SolidQueue.on_start { start_metrics_server }
SolidQueue.on_stop { stop_metrics_server }

You can do the same for workers, with registered blocks being called before the worker starts polling (worker_start) and right before it shuts down its thread pool (worker_stop):

SolidQueue.on_worker_start { AnotherExtension.stopping = false }
SolidQueue.on_worker_stop { AnotherExtension.stopping = true }

Any errors raised by these blocks will be passed over to on_thread_error and won't be re-raised.