RFR: 8326820: Metadata artificially kept alive [v4]

[Axel Boldt-Christmas]

> ClassLoaderDataGraph provides APIs for walking different metadata. All the iterators which are not designed to be used by the GC also keep the holder of the CLDs alive and by extensions keeps all metadata alive. This is problematic for concurrent GC as it keeps otherwise unreachable classes from being unloaded and the respective metadata freed. 
> 
> This patch changes the default iteration behaviour to not keep the holder alive, with the exception of `loaded_classes_do` (renamed `loaded_classes_do_keepalive`) and `modules_do` (renamed `modules_do_keepalive`) which is used by jvmti APIs that requires that the holder is kept alive.
> 
> All other uses consumes all the metadata it queries during its safepoint or before releasing the `ClassLoaderDataGraph_lock`. 
> 
> Before this change some jcmd, new jfr chunks and some jfr events, all of which consumed these APIs, could cause class unloading to not occur. 
> 
> Been running our internal stress test in an even more stressful mode which without this patch reproduces the metaspace OOME [JDK-8326005](https://bugs.openjdk.org/browse/JDK-8326005) consistently within a few hours. And after this patch it does not.
> 
> Currently running tier1-tier8 testing.

Axel Boldt-Christmas has updated the pull request incrementally with five additional commits since the last revision:

 - Rename iterator
 - Add SystemDictionary Comment
 - Revert "Rename and comment SystemDictionary::methods_do"
   
   This reverts commit 5f29cfbe79a741552683ff491360a0c920315747.
 - Revert "Fixup comments after renaming"
   
   This reverts commit 08366b1244775e5892bbbb184660821e8774f37a.
 - Revert "Rename and comment SystemDictionary::methods_do"
   
   This reverts commit 5f29cfbe79a741552683ff491360a0c920315747.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19769/files
  - new: https://git.openjdk.org/jdk/pull/19769/files/5f29cfbe..86bb89b0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19769&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19769&range=02-03

  Stats: 74 lines in 23 files changed: 0 ins; 2 del; 72 mod
  Patch: https://git.openjdk.org/jdk/pull/19769.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19769/head:pull/19769

PR: https://git.openjdk.org/jdk/pull/19769