RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v3]

[Alex Menkov]

> The fix updates JVMTI FollowReferences implementation to report references from virtual threads:
> - added heap scanning to report unmounted vthreads;
> - stacks of mounted vthreads are splitted into 2 parts (vittual thread stack and carrier thread stack), references are reported with correct thread id/class and object tags/frame depth;
> - common code to handle stack frames are moved into separate class;

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

  Fixed test
  
  - replaced obsolete java.util.concurrent.ForkJoinPool.common.parallelism with jdk.virtualThreadScheduler.parallelism;
  - added check that vthreads are mounted/unmounted;
  - disabled testing of JNI locals for unmounted thread as native call pins vthread and does not allow it to unmount.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13254/files
  - new: https://git.openjdk.org/jdk/pull/13254/files/8108f217..841f5a78

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13254&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13254&range=01-02

  Stats: 45 lines in 1 file changed: 30 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/13254.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13254/head:pull/13254

PR: https://git.openjdk.org/jdk/pull/13254