RFR: 8252104: parallel heap inspection for ShenandoahHeap [v5]

[Lin Zang]

> - enable parallel heap inspecton for ShenandoahHeap
>   - preliminary evaluation:
>     Time of jmap histo on (8GB heap with 4GB objects)
>     * before: 5.186s
>     * after : 1.698s

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  8252104: parallel heap inspection for ShenandoahHeap
  
    - enable parallel heap inspecton for ShenandoahHeap
    - preliminary evaluation:
      Time of jmap histo on (8GB heap with 4GB objects)
      * before: 5.186s
      * after : 1.698s

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/67/files
  - new: https://git.openjdk.java.net/jdk/pull/67/files/1fddea7c..a1b6a76c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=67&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=67&range=03-04

  Stats: 10 lines in 2 files changed: 1 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/67/head:pull/67

PR: https://git.openjdk.java.net/jdk/pull/67