sbws keeps the number of AsyncResults less than the number of threads

This ticket is a follow-up to legacy/trac#28864 (moved).

multiprocessing.Pool can accept any number of queued AsyncResults: https://docs.python.org/3/library/multiprocessing.html#multiprocessing.pool.Pool

But sbws waits for 5 seconds when there are max_pending_results queued AsyncResults: https://github.com/torproject/sbws/blob/ee64d76df54ceb3a3c9e1e2a797fd70d68bb0035/sbws/core/scanner.py#L359-L361

This sbws code is unnecessary, because Pool manages its own queue of AsyncResults.

There are two different ways that this code blocks execution:

• when a result finishes, the time.sleep(5) call blocks the thread from getting a new AsyncResult for up to 5 seconds
• if max_pending_results AsyncResults ever block, the process hangs