feat(uvloop): Support event loop utilization queries

[cphoward]

This change seeks to expose libuv's internal uv_metrics_idle_time to permit users to tune their applications. This change is inspired by Nodejs' internalEventLoopUtilization.

The heart of this change is:

def_event_loop_utilization(self):
"""Returns idle and active time in milliseconds and the percentage of time the event loop is active """
...
returnidle, active, utilization

[ericbuehl]

I could see this being immensely useful in support of #322

@1st1@fantix what do we think here?