RFR: 8309899: Rename PtrQueueSet::buffer_size()

[Kim Barrett]

Please review this renaming of PtrQueueSet::buffer_size to buffer_capacity,
along with some associated function and variable renamings.

Despite the number of files and lines changed, it's just a name change, so
perhaps it's trivial?

Testing:
mach5 tier1

Note: There weren't any changes needed in Shenandoah, even though it uses the
SATBMarkQueue stuff.

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Commit messages:
 - rename buffer_size to buffer_capacity

Changes: https://git.openjdk.org/jdk/pull/14439/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309899
  Stats: 68 lines in 12 files changed: 1 ins; 2 del; 65 mod
  Patch: https://git.openjdk.org/jdk/pull/14439.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14439/head:pull/14439

PR: https://git.openjdk.org/jdk/pull/14439