RFR: 8252221: Use multiple workers for Parallel GC pre-touching [v2]

[Amit Pawar]

> Please review this change.

Amit Pawar has updated the pull request incrementally with one additional commit since the last revision:

  Code is updated based on feedback and will post for final review.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/180/files
  - new: https://git.openjdk.java.net/jdk/pull/180/files/f4d0cefa..232adc93

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=180&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=180&range=00-01

  Stats: 233 lines in 5 files changed: 109 ins; 113 del; 11 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