RFR (M): 8247819: G1: Process strong OopStorage entries in parallel

[Thomas Schatzl]

Hi all,

   can I have reviews for this follow-up change to JDK-8248132, adding 
parallel OopStorage strong root processing for G1?

The main difference to other collectors is that G1 has some 
per-OopStorage timing, so a slightly different approach in iterating 
over the OopStorages has been taken. Also, this messes up G1GCPhaseTimes 
a bit (more), but I see fixing that, as it has been a mess before, a 
separate CR.

Also since the internal names of OopStorage (e.g. "VM global") are now 
used in some log messages, I upper-cased them (ie. "VM Global") to match 
other, existing log messages.

This work is based on a POC from Erik Österlund, crediting him for that.

CR:
https://bugs.openjdk.java.net/browse/JDK-8247819
Webrev:
http://cr.openjdk.java.net/~tschatzl/8247819/webrev/
Testing:
tier1-5

Thanks,
   Thomas