RFR: 8270014: Add scoped objects for g1 young gc verification and young gc internal timing

[Thomas Schatzl]

Hi all,

  can I get reviews for some further cleanup of the young collection code:
- adding some more scoped objects (verification, gc pause timing)
- some naming, minor related code movement

Thanks,
  Thomas

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Commit messages:
 - Initial implementation: further refactoring

Changes: https://git.openjdk.java.net/jdk/pull/4768/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4768&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270014
  Stats: 76 lines in 6 files changed: 38 ins; 12 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4768.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4768/head:pull/4768

PR: https://git.openjdk.java.net/jdk/pull/4768