RFR: JDK-8322042: HeapDumper should perform merge on the current thread instead of VMThread [v2]

[Alex Menkov]

> The fix updated HeapDumper to always perform merge on the current thread.
> 
> Testing: tier1-5, all HeapDump-related tests
>   Covered heap dumping scenarios:
>     - `jcmd GC.heap_dump` command;
>     - `HotSpotDiagnosticMXBean.dumpHeap()`;
>     - `HeapDumpBeforeFullGC`, `HeapDumpAfterFullGC` VM options;
>     - `HeapDumpOnOutOfMemoryError` VM option.

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18571/files
  - new: https://git.openjdk.org/jdk/pull/18571/files/e373d793..2eb802f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18571&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18571&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18571.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18571/head:pull/18571

PR: https://git.openjdk.org/jdk/pull/18571