RFR: 8159979: During initial mark, preparing all regions for marking may take a significant amount of time

[Ivan Walulya]

Hi all,

Please review this change to move the work done during preparation for a concurrent mark start (pre_concurrent_start) into a G1BatchedGangTask.

Testing: Tier 1-3

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Commit messages:
 - 8159979: During initial mark, preparing all regions for marking may take a significant amount of time

Changes: https://git.openjdk.java.net/jdk/pull/5302/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5302&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8159979
  Stats: 121 lines in 5 files changed: 87 ins; 20 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5302.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5302/head:pull/5302

PR: https://git.openjdk.java.net/jdk/pull/5302