RFR: 8247820: ParallelGC: Process strong OopStorage entries in parallel

[Stefan Karlsson]

Hi all,

Please review this patch to both unify handling of all OopStorage 
instances and parallelize it in the root processing of the Parallel GC.

https://cr.openjdk.java.net/~stefank/8247820/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8247820

This removes the explicit enumeration of "strong" OopStorages in the 
Parallel GC. This is a step towards allowing the Runtime code to add new 
OopStorages without having to update all GCs.

It also parallelizes the processing of the OopStorages, using the class 
that's being introduced in:
https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-June/030152.html

Tested with tier1-3

Thanks,
StefanK