Instrumenting threadpool more systematically

This branches off https://github.com/rackerlabs/blueflood/pull/383 so it should be merged afterwards.

This PR changes `InstrumentedThreadPoolExecutor` dramatically. Instead of making it a subclass of `ThreadPoolExecutor`, the instrumentation is done via static method. this allows us to instrument any `ThreadPoolExecutor` free of class hierarchy pollution, and specifically allows us to use `ThreadPoolBuilder` more often.

I added default instrumentation to the `ThreadPoolBuilder`. this means that the all the threadpools within blueflood are instrumented by default, and exports coda hale metrics to whichever system it is exporting the metrics to.