RFR: 8265842: G1: Introduce API to run multiple separate tasks in a single gangtask

[Thomas Schatzl]

JDK-8214237 is going to join different somewhat independent (sub-)tasks of the post evacuation phase into basically two gangtasks to improve performance (reduce startup and shutdown of work gangs).

To facilitate doing that, introduce an API that manages subtasks of a given gangtask and provide some helpers to do common work.

This CR is only about adding the API and testing to somewhat simplify the actual move of the post evacuate tasks into parallel tasks. There is no new functionality just to split up JDK-8214237 a bit.

Here is the full change that uses this API to implement JDK-8214237 (which I'll split up a bit before) to see how it will be used (in G1YoungGCPostEvacuateTasks.hpp.

Testing: new gtest, tier1-5 with mentioned patch for JDK-8214237.

Thanks,
Thomas

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Commit messages:
 - Whitespace fixes
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/3653/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3653&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265842
  Stats: 386 lines in 3 files changed: 386 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3653.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3653/head:pull/3653

PR: https://git.openjdk.java.net/jdk/pull/3653