RFR(XS) 8212074: Add method to peek the remaining tasks in task queues

[Zhengyu Gu]

Hi All,

Shenandoah implemented an variant of termination protocol based on [1], 
which requires to peek into task queues for remaining tasks, so that it 
can wake just enough workers to steal tasks.

The patch adds methods for this purpose, and allow subclasses of 
ParallelTaskTerminator to access it members.

Bug: https://bugs.openjdk.java.net/browse/JDK-8212074
Webrev: http://cr.openjdk.java.net/~zgu/8212074/webrev.00/

Test:

   hotspot_gc on Linux 64 (fastdebug and release)

Thanks,

-Zhengyu

[1] Understanding and improving JVM GC work stealing at the data center 
scale (http://dl.acm.org/citation.cfm?id=2926706)