RFR: 8277757: Parallelize humongous reclaim

[Stefan Johansson]

Please review this change to use parallel threads for humongous reclaim.

**Summary**
The task doing humongous reclaim is part of "Post Evacuate Cleanup 2", but currently it is executed like a serial task. In many cases this is not a problem, but if the humongous reclaim is the only part in cleanup 2 that has substantial work, it would benefit from using multiple threads. There is no technical problem with this and this change simply enables multiple threads to do the work in parallel. 

**Testing**
- [x] Mach5 tier 1-3
- [x] Local testing verifying JFR tests are unaffected
- [x] Performance testing checking for regressions

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Commit messages:
 - 8277757: Parallelize humongous reclaim

Changes: https://git.openjdk.java.net/jdk/pull/6612/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6612&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277757
  Stats: 27 lines in 2 files changed: 13 ins; 3 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6612.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6612/head:pull/6612

PR: https://git.openjdk.java.net/jdk/pull/6612