RFR: 8322043: HeapDumper should use parallel dump by default [v6]

[Alex Menkov]

> The fix makes VM heap dumping parallel by default.
> `jcmd GC.heap_dump` and `jmap -dump` had parallel dumping by default, the fix affects `HotSpotDiagnosticMXBean.dumpHeap()`, `-XX:+HeapDumpBeforeFullGC`, `-XX:+HeapDumpAfterFullGC` and `-XX:+HeapDumpOnOutOfMemoryError`.
> 
> Testing:
>   - manually tested different heap dump scenarios with `-Xlog:heapdump`;
>   - tier1,tier2,hs-tier5-svc;
>   - all reg.tests that use heap dump.

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

  removed space-only change in diagnosticCommand.cpp

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18748/files
  - new: https://git.openjdk.org/jdk/pull/18748/files/641bedc5..eb38705d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18748&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18748&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18748.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18748/head:pull/18748

PR: https://git.openjdk.org/jdk/pull/18748