RFR: 8245022: ParallelGC abuses StarTask to also include partial objarray scan tasks

[Kim Barrett]

Please review this change to ParallelGC to also use the recently added
ScannerTask (JDK-8244684), eliminating some casts between oop and oop*
and use of some misaligned oop* values.

Also renamed G1's ScannerTasksQueue[Set] to G1ScannerTasksQueue[Set].
I should have used those names in JDK-8244684.  This is consistent
with the new PSScannerTasksQueue[Set] now being used by ParallelGC.
The types used by G1 and Parallel are currently the same, but don't
need to be.

Also made a small simplification, reordering the template parameters
for PSPromotionManager::copy_and_push_safe_barrier.  By making the
promote_immediately flag first and the reference type second we can
explicitly specify the non-deduced flag and allow the reference type
to be deduced, rather than having to specify both explicitly in calls.

CR:
https://bugs.openjdk.java.net/browse/JDK-8245022

Webrev:
https://cr.openjdk.java.net/~kbarrett/8245022/open.00/

Testing:
mach5 tier1-5
No significant differences in ParallelGC performance testing, including specjbb2015.