RFR (S) 8135152: Create a G1ParScanThreadStateSet class for managing G1 GC per thread states

[Mikael Gerdin]

Hi all,

I'd like to move management of the G1ParScanThreadState instances into a 
ParScanThreadStateSet class, similar to how ParNew works.

This first change simply moves the code for allocation/deallocation
and flushing of stats from the par scan thread state to the state set 
object.

There is another change coming (8135154) where some additional stats and 
counters are moved from other ad-hoc locations to the par scan thread 
state set.

Webrev: http://cr.openjdk.java.net/~mgerdin/8135152/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8135152

Testing: JPRT

/Mikael