RFR: 8302206: Factor out duplicate G1VerificationClosure

[Thomas Schatzl]

Hi all,

  can I have reviews for this cleanup that merges `G1VerifyLiveClosure` from `g1FullGCMarker.cpp` and `VerifyLiveClosure` from `heapRegion.cpp` into a single one as they are virtually identical.

There is probably one change that needs some discussion, and that is about changing the type and meaning of `G1MaxVerifyFailures`: after changing the type of `G1VerificationClosure::_num_failures` to uint, the given comparison does not work any more due to differing types.
However the change of the default value to `0` meaning "show all messages" does not seem too bad: even before this change, a value of `0` at least showed one failure as this check (now it shows all messages) is located after the error printing in the closures.
It's a develop flag too, so instead of casting around I chose to simply change the meaning.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - Fix compile error with G1MaxVerifyFailures
 - initial attempts, nothing done

Changes: https://git.openjdk.org/jdk/pull/12511/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12511&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302206
  Stats: 340 lines in 11 files changed: 114 ins; 188 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/12511.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12511/head:pull/12511

PR: https://git.openjdk.org/jdk/pull/12511