RFR: Force termination logic is inverted

[Zhengyu Gu]

When should_force_termination() returns true, termination should be 
terminated even there are remaining tasks. Current logic does the 
opposite, it jumps right back for stealing works, e.g. returning false.
Which means, it actually drains task queues, instead of leaving tasks 
there for degen cycle.

Weberev: 
http://cr.openjdk.java.net/~zgu/shenandoah/force_termination/webrev.00/

Test:

   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu