Crochet's shutdown strategy is problematic

[itamarst]

1. A thread named MainThread doesn't always exist (see fix crochet crashing when MainThread isn't available when _shutdown module is loaded #69, some comments crochet doesn’t work with uWSGI #78).
2. Shutting down when the main thread exits may be the wrong thing to do; perhaps the main thread starts a thread pool of workers and then exits.

Perhaps it would be better to just set all the crochet threads to be daemon threads.
Downsides:

1. No reactor shutdown events.
2. An occasional segfault? Though maybe modern Python is better than that.

[itamarst]

Combination of #83 and #81 are going to fix this, I believe.