Audit glog logging levels, make them consistent, and document our policy.

[ZackButcher]

We have a variety of glog logging levels throughout the Mixer codebase. Generally, glog.V(2) is what we consider default logging level, and glog.V(4) is what we consider verbose. However, some portions of the codebase use glog.V(3), some some use glog.V(9). It's likely that even more levels are used.

We need to audit our glog calls to determine where we log what information and at what verbosity. After that we should decide on a standard to follow throughout the codebase. Then we need to go update logging everywhere to conform to that standard.

[geeknoid]

Part of the solution for this is #1489