In Python 3.6-3.9, unlike in Python 2, co_lnotab's line number offsets can be negative and are represented by a signed integer. The Debugger agent had been using an unsigned integer which prevented it from being able to set breakpoints on lines of code in a code object after the line table had a negative offset.

This PR also enables most of the previously disabled tests now that they work on all supported versions of Python. A couple still remain disabled and will be fixed later on.

- The firebase backend client code now calls on_idle periodically
- The breakpoint expiration code now checks for the `createTimeUnixMsec` breakpoint field

Fixes #72

The emitted logs when the DB does not exist will now resemble:

```
I0221 20:25:54.661298 93984 firebase_client.py:402] Failed to check debuggee presence at cdbg/debuggees/d-93910f74/registrationTimeUnixMsec: NotFoundError('404 Not Found')
I0221 20:25:54.661362 93984 firebase_client.py:373] registering at https://my-project-cdbg.firebaseio.com, path: cdbg/debuggees/d-93910f74
I0221 20:25:54.690697 93984 firebase_client.py:390] Failed to register debuggee: NotFoundError('404 Not Found')
```

Fixes #77

Fixes #79