Show the current thread name in the log file.

This is very useful, although it kinda entwines the logging a bit
more than might be desired.
Add a VERBOSE logging level, for noisy logging.

The debug log is hard to look at because it has so much unneeded info.
Turn down logging for things that can be inferred.

If you see a thread switch for a vblank wait, you know what happened.
Turn down logging for LwMutexes.

No one seems to have run into issues with these for a while, and they are
quite noisy, since games use them as lightweight locks.
Fix logging during savestate.
Also clear the logging thread name on shutdown.
Merge pull request #940 from unknownbrackets/logging

Add a VERBOSE logging level + show thread name in log