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

[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.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/18571/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18571&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322042
  Stats: 26 lines in 2 files changed: 0 ins; 22 del; 4 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