RFR: Empty task queues when cancelled partial GC

[Roman Kennke]

When partial GC gets cancelled, we need to empty the task queues.

I also added a small check to verify this, even for release build
(shuold we make this debug-only?)

http://cr.openjdk.java.net/~rkennke/fixemptyq/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/fixemptyq/webrev.00/>

Ok?