RFR: 8293623: Simplify G1ConcurrentRefineThreadControl [v4]

[Lei Zaakjyu]

> 8293623: Simplify G1ConcurrentRefineThreadControl

Lei Zaakjyu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into JDK-8293623
 - modify class 'G1ConcurrentRefineThreadControl'
 - use 'append'
 - use 'mtGC' flag
 - simplify 'G1ConcurrentRefineThreadControl'

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17077/files
  - new: https://git.openjdk.org/jdk/pull/17077/files/25aed5b5..5c14ca7e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17077&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17077&range=02-03

  Stats: 13559 lines in 229 files changed: 10584 ins; 2080 del; 895 mod
  Patch: https://git.openjdk.org/jdk/pull/17077.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17077/head:pull/17077

PR: https://git.openjdk.org/jdk/pull/17077