RFR 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[Thomas Schatzl]

Hi Zhengyu,

On Fri, 2018-11-30 at 10:00 -0500, Zhengyu Gu wrote:
> Hi Thomas,
> 
> Thanks for testing and cleaning it up.

  I think that change is good.

What I was trying to do was getting rid of the assignment operator in
TaskTerminator, because of its semantics. It's a "move", not an
assignment.

So I created the change at

http://cr.openjdk.java.net/~tschatzl/8204947/webrev.1_to_2

which does that, and simply provides a reset_for_reuse() method which
should do the same.

However, when trying this out I rarely get an assertion failure in
taskqueue.cpp:234.

>From what I understand, the effect of both variants should be the same,
so I would like to investigate this further. So far it looks like a
(pre-existing, benign?) G1 bug.

Thanks,
  Thomas