... and drop a few unnecessary references to it

move `current_context` and `set_context` out to top-level functions.

Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.

... what even is the point of it.

`alive` is dead.

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

Rather than killing off `alive` altogether, turn it into `finished` and make it a bit better defined.