RFR (S): 8144714 Add extension point to G1 evacuation closures

[Mikael Gerdin]

Hi all,

Please review this small change to add a hook in the G1 evacuation path.
This allows extension code to register regions which are otherwise 
interesting to the GC and get notifications when pointers into such 
regions are encountered.

Testing: JPRT, gc test suite
Performance:
This additional branch in G1ParScanClosure introduces a slight 
regression of about 1-2% in GC pause times. I have some performance 
fixes to make up for that regression in the pipe.

webrev: http://cr.openjdk.java.net/~mgerdin/8144714/webrev.0/
bug: https://bugs.openjdk.java.net/browse/JDK-8144714

Thanks
/Mikael