Monitor main_thread in Database/AsyncDatabase to determine whether we need to automatically call close()

[blast-hardcheese]

Why

Especially with default_db, it's not clear that the user still needs to call close(). Monitor the main thread for termination to do this automatically.

What changed

The presumption is that users won't be relying on the database to hold the python runtime open, and that any instance of that is in fact some form of error.

Test plan

>>> from replit.database import db
>>> ^D

confirm that we exit cleanly and quickly.

Rollout

No special instructions.

• This is fully backward and forward compatible

[blast-hardcheese]

Further testing revealed that the way we do threading doesn't work with atexit. Trying to find a workaround now.

[blast-hardcheese]

Ok. New approach.

Spawn a watchdog that monitors main_thread() so we know when to automatically call self.close().

[lincoln-replit]

Some kind of test for this would be awesome but otherwise LGTM!