RFR: 8317692: jcmd GC.heap_dump performance regression after JDK-8292818

[Hannes Greule]

See the bug description for more information.

This implementation brings down the time to take a heap dump on the example application in the bug report to <2 seconds on my machine.

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Commit messages:
 - whitespaces
 - Iterate fields forwards on thread dump

Changes: https://git.openjdk.org/jdk/pull/16083/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16083&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317692
  Stats: 110 lines in 2 files changed: 102 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/16083.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16083/head:pull/16083

PR: https://git.openjdk.org/jdk/pull/16083