Easier & cleaner logging

[ulugbekna]

Sets up easier logging facility without passing around the server or a logger callback. Very much a proof of concept.

@rgrinberg What do you think of such a logger? It seems to make logging easier and cleaner. Kind of does what #421 says (?)

I am trying to improve logging generally, which would also include adding support for merlin logs.

[rgrinberg]

What do you think of such a logger? It seems to make logging easier and cleaner. Kind of does what #421 says

It's not quite the same. #421 suggests a solution to send logs to the client when outside the fiber. As far as I can tell, this PR does not address this limitation.

The rest of the changes seem OK to me. However, instead of using a global variable you should use a fiber variable to store the logger instance.