RFR: 8300653: G1EvacInfo should use common naming scheme for collection set

[Thomas Schatzl]

Hi all,

  please review this imo trivial renaming of some identifiers containing `collectionset` to `collection_set`.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/12096/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12096&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300653
  Stats: 24 lines in 4 files changed: 0 ins; 0 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/12096.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12096/head:pull/12096

PR: https://git.openjdk.org/jdk/pull/12096