feat(uvloop): Support event loop utilization queries

[caseydialpad]

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
    """

    ...

    return idle, active, utilization

[caseydialpad]

Tests will be fixed by a change like #512.