[Spike] Proof of concept for modular logging

[ccressent]

This shows a way to make logging more modular, in the spirit of #3809 and #4415.

I've included comments in the change to explain how that works and some of the issues we would need to address (e.g. the live log level changing facility with signals).

Only apid and authentication have been modified to demonstrate the approach.

This was only tested manually, playing with different --log-level apid:<level>,authentication:<level> flags and using sensuctl to log in and out and list various object types, while looking at the backend logs.

While not tested extensively, a log level with no component prefix is meant to be interpreted as the default, global log level to use for everything else that's not more finely tuned. For example, --log-level error,authentication:debug would mean that we want everything to be at log level error, with the exception of the authentication module being set to debug.

Note that we can use a very similar approach to configure different outputs for different components too. That could pave the way towards a cheap, wannabe "audit log" with a configuration like --log-output authentication:/var/log/sensu/audit.log,authorization:/var/log/sensu/audit.log, with everything else on stderr by default.