RFR: 8189733: Cleanup Full GC setup and tear down

[Stefan Johansson]

Hi,

Please review this enhancement:
https://bugs.openjdk.java.net/browse/JDK-8189733

Webrev:
http://cr.openjdk.java.net/~sjohanss//8189733/00/index.html

Summary:
After the G1FullCollector has been introduced as part of JEP 307 it 
makes sense to move more of the setup and tear down for the Full GC into 
this class. The G1FullGCScope is also moved into the G1FullCollector 
since it is now only used here.

Testing:
* Tier 1-2

Thanks,
Stefan