Introduce a new logging package

[geeknoid]

We want to create a new logging package for use in Mixer, and eventually Istio-wide. This package would overlay on top of glog and embed standard Istio policies. For example, this would capture specific log levels.

A side goal would be to create a layer that reduces the overhead where we can for logging code by providing faster early exits in the case of disabled logging statements.

This issue also tracks the conversion of the Mixer codebase to use this new package.

[geeknoid]

There's a PR out for a prototype of this.

[ldemailly]

aforementioned PR is istio/istio#1840