AIM: add early check for log level

This speeds up my barrier benchmark by 20%. It compiles down to a "testb; je"
which skips all of the argument setup for the varargs call (including things
like strerror) and the call itself.

I disabled this if the environment variable check is on so that I don't break
that feature. It seems like the env lookups could be done at log registration
time, which would make them compatible with this optimization.
Merge into master from pull request #20:
AIM: add early check for log level (https://github.com/floodlight/infra/pull/20)