RFR: 8319439: Move BufferNode from PtrQueue files to new files

[Kim Barrett]

Please review this simple (trivial?) change that moves the BufferNode class
from share/gc/shared/ptrQueue.[ch]pp to new files
share/gc/shared/bufferNode.[ch]pp.  The code (as opposed to the surrounding
boiler plate and #include's) was moved unchanged.

This led to adding #include's of the new header file to a bunch of other files
(include what you use), in some cases eliminating the need to #include
ptrQueue.hpp.  Also tidied up the #include's for ptrQueue.[ch]pp to take
advantage of the split.

Also, the gtest gc/shared/test_ptrQueueBufferAllocator.cpp is renamed to  
gc/shared/test_bufferNodeAllocator.cpp, with some adjustments to includes and
test naming.

Testing:
mach5 tier1

-------------

Commit messages:
 - fixup gtest after rename
 - move test
 - move BufferNode to new files

Changes: https://git.openjdk.org/jdk/pull/16506/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16506&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319439
  Stats: 383 lines in 17 files changed: 223 ins; 152 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/16506.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16506/head:pull/16506

PR: https://git.openjdk.org/jdk/pull/16506