RFR: 8291912: Introduce per-allocation target struct for members in G1PLABAllocator

[Thomas Schatzl]

Hi all,

  can I get reviews for this change that refactors the many per-destination arrays in PLABAllocator into an array of struct as I promised in the review for [JDK-8288966](https://bugs.openjdk.org/browse/JDK-8288966)/https://github.com/openjdk/jdk/pull/9726? I think that makes the code easier to follow since there is less clutter.

Testing: tier1-5, no performance differences

Thanks,
Thomas

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Commit messages:
 - Merge branch 'master' into 8291912-per-allocation-target-struct
 - initial version
 - Merge branch 'master' into 8288966-plab-boost
 - iwalulya review
 - Fix whitespace
 - Only boost with ResizePLAB
 - Refactoring, fix boundless boosting
 - some refinement
 - Some cleanup
 - More removal of conditional code
 - ... and 4 more: https://git.openjdk.org/jdk/compare/7b5f9edb...54c93338

Changes: https://git.openjdk.org/jdk/pull/9959/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9959&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291912
  Stats: 115 lines in 3 files changed: 59 ins; 30 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/9959.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9959/head:pull/9959

PR: https://git.openjdk.org/jdk/pull/9959