Django configuration compatibility

[quinnmil]

Changes

• Fixes an issue where configuration from the Django settings.py was not loaded by the time the handler was initialized, which would result in failed startup.
• Now, the handler waits to initialize until the first time emit is called.
• Since the initialization process can itself emit log messages, this can cause infinite recursion, so to avoid this, we add a flag to check for an initialization in-progress to avoid the multiple init calls which are made when django sets up logging. We use a thread thread lock to further guard against recursion/race conditions.

Notes

• This is hardly the most elegant solution. The proper way to do a django logger is probably be the package to be added an an app and/or middleware. Still, this does work, and is sufficient for the time being.