RFR: 8306028: separate ThreadStart/ThreadEnd events posting code in JVMTI VTMS transitions [v4]

[Serguei Spitsyn]

> This refactoring to separate ThreadStart/ThreadEnd events posting code in the JVMTI VTMS transitions is needed for future work on JVMTI scalability and performance improvements. It is to easier put this code on slow path.
> 
> Testing: mach5 tiers 1-6 were successful.

Serguei Spitsyn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge branch 'master' into br29
 - do more refactoring including VirtualThread class
 - Merge
 - 8304444: Reappearance of NULL in jvmtiThreadState.cpp
 - 8306028: separate ThreadStart/ThreadEnd events posting code in JVMTI VTMS transitions

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

Changes: https://git.openjdk.org/jdk/pull/13484/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13484&range=03
  Stats: 331 lines in 16 files changed: 182 ins; 71 del; 78 mod
  Patch: https://git.openjdk.org/jdk/pull/13484.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13484/head:pull/13484

PR: https://git.openjdk.org/jdk/pull/13484