RFR 8031820: NPG: Fix remaining references to metadata as oops in comments

[Coleen Phillimore]

Also fixed:
8012125: Comments for ConstantPoolCache should reflect the addition of 
resolved_references in ConstantPool
Summary: Updated comments in metadata header files, and renamed this_oop 
variables to this_cp or this_k when referring to constant pool or classes.

open webrev at http://cr.openjdk.java.net/~coleenp/8031820/
bug link https://bugs.openjdk.java.net/browse/JDK-8031820

Tested with jck, all jtreg hotspot tests and nsk vm.quick.testlist.

Thanks,
Coleen