RFR: 8028993: Full collections with ParallelScavenge slower in JDK 8 compared to 7u40

[Stefan Johansson]

Hi,

Can I have some reviews for this change to fix:
https://bugs.openjdk.java.net/browse/JDK-8028993

Webrev:
http://cr.openjdk.java.net/~sjohanss/8028993/webrev.00/

Summary:
While fixing JDK-8027675 we realized that we should have a similar 
regression with ParallelGC. The regression is not as big as with serial 
but the logic changes are the same and I've been able to measure a 
significant improvement with this fix, taking us on par with 7u40.

The theory behind the fix is the same as when fixing serial:
---
To improve the mark phase:
* Enabled the calls to follow_klass to be inlined.
* Reduce the extensive amount of calls to follow_class_loader in 
follow_klass and instead just mark the klass holder (the class loader or 
the java mirror) for the given klass.

To improve the adjust phase:
* All calls to adjust_klass have been removed since this is already 
handled by processing the ClassLoaderDataGraph.
---

Testing:
* Building and basic testing through JPRT
* Some manual testing running Nashorn tests
* Performance testing with SPECjbb2005

Thanks,
Stefan