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 preparing regions for marking to the multithreaded execution in G1PrepareEvacuationTask. Additionally, we add more marking details to the humongous region logs.

Testing: Tier 1-3.

-------------

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

Changes: https://git.openjdk.java.net/jdk/pull/5251/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5251&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8159979
  Stats: 19 lines in 2 files changed: 6 ins; 12 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5251.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5251/head:pull/5251

PR: https://git.openjdk.java.net/jdk/pull/5251