deadlock if send_stop_tokens() only sends stop tokens for #-alive-processes

[gatoatigrado]

this is reproducible via test_fail_after_a_while(). I'm not sure what's going on yet. At first I thought perhaps processes hadn't yet started, but this doesn't seem to be the case -- new assertions in fork() show that processes are marked as is_alive() immediately after their start() method is called.