RFR: 8243146: Further cleanups after UseAdaptiveGCBoundary removal

[Stefan Karlsson]

Hi all,

Please review this patch to further cleanup after the 
UseAdaptiveGCBoundary removal.

https://cr.openjdk.java.net/~stefank/8243146/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8243146

Changes in the patch:
- Remove AdjoiningGenerations and inline the contents into 
ParallelScavengeHeap::initialize.
- Remove AdjoiningVirtualSpaces.
- Remove PSVirtualSpaceHighToLow
- Update PS{Old,Young}Gen:
  -- Move initialization to constructors, and made the functions private.
  -- Remove unused functions
  -- Change protected to private
  -- Remove most virtual declarations
- WB_PSVirtualSpaceAlignment - inlined the old value. There seems to be 
some, arguably, odd usage of GenAlignment where SpaceAlignment seems 
more appropriate. They both contain the same value, but it looks weird. 
Maybe something to fix in a separate RFE.

Thanks,
StefanK