RFR: 8078193: BACKOUT: Rename and clean up the ParGCAllocBuffer class

[Kim Barrett]

Please review this change to back out a changeset.

The intended fix for JDK-8074546 was pushed with a commit message
containing the wrong bug id. To simplify tracking the changeset
through repository promotions to release, we're going to hg backout
the problem changeset and re-push with a corrected commit message.

The hg backout applied cleanly:
Changeset: hs-gc/hotspot: 8148:dafb0a98e749 

CR:
https://bugs.openjdk.java.net/browse/JDK-8078193

Webrev:
http://cr.openjdk.java.net/~kbarrett/8078193/webrev.00/

Testing:
JPRT