RFR: 8252221: Use multiple workers for Parallel GC pre-touching

[Amit Pawar]

Please review this change.

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Commit messages:
 - 8252221: Improves JVM startup time for ParallelGC Pretouch using multiple threads

Changes: https://git.openjdk.java.net/jdk/pull/180/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=180&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252221
  Stats: 99 lines in 7 files changed: 82 ins; 4 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/180.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/180/head:pull/180

PR: https://git.openjdk.java.net/jdk/pull/180