RFR: 8273559: Shenandoah: Shenandoah should support multi-threaded heap dump [v2]

[Zhengyu Gu]

> At the point when Shenandoah implemented heap dump, there was no ClassLoaderData::_claim_other flag. To avoid CLDG iteration interfering concurrent GC, we chosen single-threaded CLDG iteration with ClassLoaderData::_claim_none and ensured that by asserting calling thread has to be VMThread.
> 
>  JDK-8237354 broke the assumption, as it uses safepoint worker to execute heap dump, so that the thread executes CLDG iteration is not necessary VMThread.
> 
> Now, with new ClassLoaderData::_claim_other, we can enable multi-threaded CLDG iteration during heap dump.
> 
> Test:
> 
> - [x] hotspot_gc_shenandoah
> - [x] tier1 with Shenandoah GC

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Aleksey and Severin's comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5473/files
  - new: https://git.openjdk.java.net/jdk/pull/5473/files/1432e964..31406df7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5473&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5473&range=00-01

  Stats: 12 lines in 5 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5473.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5473/head:pull/5473

PR: https://git.openjdk.java.net/jdk/pull/5473