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

[Yi Yang]

On Tue, 2 Apr 2024 00:40:37 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

> 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.

- jcmd GC.heap_dump command; `AttachListenerThread`
- HotSpotDiagnosticMXBean.dumpHeap(); `JavaThread`
- HeapDumpBeforeFullGC, HeapDumpAfterFullGC VM options; `VMThread`
- HeapDumpOnOutOfMemoryError VM option. `VMThread`
Mabye we can always use AttachListenerThread(via Handshake) or new virtual thread?

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PR Review: https://git.openjdk.org/jdk/pull/18571#pullrequestreview-1972484174