RFR: 8290525: Move HeapRegion::_compaction_top to G1FullCollector

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that moves `HeapRegion::_compaction_top` into the `G1FullCollector` class; my reasoning is that these values are transient temporary values only ever useful for the full collection algorithm, basically results of the application of the algorithm, that are otherwise unused. This makes `HeapRegion` a bit easier to understand/read.

Testing: tier1-3

Thanks,
  Thomas

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Commit messages:
 - fix copyright years
 - Fix verification - needs to be uninitialized (nullptr)
 - initial version

Changes: https://git.openjdk.org/jdk/pull/9560/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9560&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290525
  Stats: 79 lines in 14 files changed: 28 ins; 32 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/9560.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9560/head:pull/9560

PR: https://git.openjdk.org/jdk/pull/9560