RFR: JDK-8316691: Heap dump: separate stack traces for mounted virtual threads

[Alex Menkov]

This is subtask of JDK-8299426: Heap dump does not contain virtual Thread stack references
The change:
- reorganize thread-related code/prepare it to use for unmounted vthreads;
- separate track traces for mounted virtual threads:
  - separate HPROF_FRAME/HPROF_TRACE records for mounted vthreads and carrier threads;
  - separate HPROF_GC_ROOT_THREAD_OBJ/HPROF_GC_ROOT_JAVA_FRAME/HPROF_GC_ROOT_JNI_LOCAL subrecords;
- updated hprof parser test lib to collect data about threads (HPROF_GC_ROOT_THREAD_OBJ subrecords).

Testing - tier1-tier3, new test

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Commit messages:
 - copyright in hprof test lib
 - separate stack traces for mounted VT

Changes: https://git.openjdk.org/jdk/pull/15869/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15869&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316691
  Stats: 887 lines in 6 files changed: 689 ins; 137 del; 61 mod
  Patch: https://git.openjdk.org/jdk/pull/15869.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15869/head:pull/15869

PR: https://git.openjdk.org/jdk/pull/15869