RFR: 8057658: Enable G1 FullGC extensions

[Stefan Johansson]

Hi,

Please review this change for RFE:
https://bugs.openjdk.java.net/browse/JDK-8057658

Webrev:
http://cr.openjdk.java.net/~sjohanss/8057658/webrev.00/

Summary:
Refactored the G1 mark sweep code to enable specialized FullGC 
implementations. Also prepared for doing context specific FullGC related 
to the work for JDK-8057536.

Testing:
* JPRT
* Manual sanity testing

Thanks,
Stefan