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

[Thomas Schatzl]

Hi Zhengyu,

On Fri, 2018-10-12 at 09:28 -0400, Zhengyu Gu wrote:
> 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/

  could you please move line 451 of taskqueue.hpp in the obviously
following patch that adds the ShenandoahTaskTerminator to the
Shenandoah specific code? It does not seem to fit here.

Otherwise it looks good. Btw, feel free to take over JDK-8204947 - I
just did not get the time due to other more important changes.

Thanks,
  Thomas