Magic Folder logging

[meejah]

All of the magic-folder logging goes through QueueMixin._log which is a call-through to Node.log, which just calls-through to allmydata.util.log, which assigns msg as foolscap.logging.log which is really the .msg of a singleton FoolscapLogger instance.

...but, I can't see how they're ever emitted (e.g. there's no observers set up in foolscap's FoolscapLogger?).

Much of the logging should also be given a level=NOISY kwarg so that it only shows up during debug etc.

All that said, it would be nice to use the new Twisted structured-ish-logging APIs which has the benefit of delaying string-interpolation as long as possible (which may be "forever" for many cases). I don't know how or if that would affect logging from Foolscap pieces.