RFR: 8295118: G1: Clear CLD claim marks concurrently

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that moves out clearing CLD marks from the concurrent start pause to the concurrent phase?

The idea is that instead of clearing CLD marks just before marking through, clear the marks at the end of the concurrent phases (or at the end of the full gc) so that after that operation marks are reset.

I believe that one can save one of the `ClassLoaderDataGraph::clear_claimed_marks` in full gc by using different claim values (we need the one at the beginning and the end though), but the overhead of that should be minimal compared to actual full gc time.

Testing: tier1-5

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/10675/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10675&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295118
  Stats: 59 lines in 9 files changed: 19 ins; 28 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/10675.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10675/head:pull/10675

PR: https://git.openjdk.org/jdk/pull/10675