RFR: 8322043: HeapDumper should use parallel dump by default

[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.

-------------

Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/18748/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18748&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322043
  Stats: 15 lines in 4 files changed: 5 ins; 0 del; 10 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