warn if there are more pending jobs than idle threads

[mmerickel]

Fixes comments in #243 (comment).

1. A warning will now be emitted if a bunch of tasks are submitted while there is an idle thread, before the threads pop the jobs, because we know that when they pop them there will still be too many.
2. Greatly simplified the shutdown logic on the threads to use only stop_count and get rid of the tombstoned None task.
3. Shutdown now uses a condition to exit every so slightly faster when all threads are complete by having each dying thread notify.
4. Added a warning on shutdown if any tasks are canceled.