Timeout and waitpid should be in side-threads

[nevinera]

Rather than the current threading approach, which will require careful thought when any change is made to the timeout code, we can move to a more explicit approach:

Process.waitpid2 and Timeout.timeout can each run in a thread - rather than the current blocking wait, we'll use Process::WNOHANG and sleep-loop, checking a queue and then trying a wait every quarter-second. When the timeout goes off, it'll go off in a thread, and push a :timeout onto the queue. This approach prevents either thread from interrupting the other, and makes adding complexity to the timeout code (as in #3) much more reasonable.

Suggestion from moomaka here.