Fix ThreadPool#shutdown timeout accuracy

[wjordan]

Description

Fixes up ThreadPool#shutdown so that the provided timeout value (which corresponds to the force_shutdown_after configuration option) is more accurate, and can be set as a float instead of just an integer value.

Previously, the shutdown timeout ran a loop timeout.times, which called t.join 1 on each thread (one after another, not parallel), followed by a sleep 1 each loop iteration, then t.join SHUTDOWN_GRACE_TIME on each thread after raising ForceShutdown. Instead of forcing shutdown in timeout (+ SHUTDOWN_GRACE_TIME) seconds, this forced a shutdown in timeout * (n + 1) (+ n * SHUTDOWN_GRACE_TIME) seconds (where n is the number of running threads). The updated code fixes this by joining a maximum of timeout seconds (followed by a maximum of grace seconds) across all threads.

I also added an extra step at the end that calls kill on any remaining threads still alive after the grace period, plus another short (1-second) join to wait for force-killed threads to (finally) exit, as an extra precaution that threads are fully cleaned up when shutdown returns. I thought this might be helpful, but could be removed as it's not strictly needed for the rest of this PR.

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[nateberkopec]

LGTM, anyone else have any thoughts?