RFR: 8306028: separate ThreadStart/ThreadEnd events posting code in JVMTI VTMS transitions [v7]

[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 10 commits:

 - Merge
 - minor correction in sharedRuntime.cpp
 - addressed review comment: add a couple of asserts
 - Merge branch 'br29' of https://github.com/sspitsyn/jdk into br29
   merge with branch29
 - Merge branch 'master' into br29
 - move code a little bit
 - 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

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Changes: https://git.openjdk.org/jdk/pull/13484/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13484&range=06
  Stats: 333 lines in 16 files changed: 184 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