Applications with many regions in the cset (large young gen) and many threads show that there is a significant amount of unaccounted "Other" times.

One part of this is caused by initializing and merging the information in all ParScanThreadStates for all threads in the VM pause.

This CR is about fixing the initialization by moving the ParScanThreadState initialization into the worker threads.

The PSS merge phase is covered by JDK-8150239.