Rework debug logging for 3.0

This branch removes TRACKER_VERBOSITY and the old logging infrastructure.

To see logs while testing, developers should set G_MESSAGES_DEBUG=Tracker in their environment.

There's a new TRACKER_DEBUG variable, similar to GTK_DEBUG which allows us to turn expensive and verbose debug output off by default, to be enabled when needed.

We now opt into GLib's structured logging support.

Fixes #178 (closed)