RFR: 8317692: jcmd GC.heap_dump performance regression after JDK-8292818 [v2]

[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.

Hannes Greule has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - whitespace
 - add test to verify all instance fields are present in heap dump
 - Merge remote-tracking branch 'upstream/master' into perf/fieldstream
 - whitespaces
 - Iterate fields forwards on thread dump

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16083/files
  - new: https://git.openjdk.org/jdk/pull/16083/files/0bcab694..63887422

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16083&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16083&range=00-01

  Stats: 19028 lines in 522 files changed: 10287 ins; 4212 del; 4529 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