RFR: Add task termination and enhanced task queue state tracking

[Aleksey Shipilev]

On 07/12/2018 03:39 PM, Zhengyu Gu wrote:
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/termination_tracking/webrev.00/

The patch looks good.

How often do threads request termination? Are we sure this tracking does not introduce its own
latencies, e.g. due timer calls and contention on counters themselves? Maybe we can guard it with
the diagnostic ShenandoahTerminationTrace flag in ShenandoahTerminationTimings to be able to dissect?

Thanks,
-Aleksey