RFR: 8306028: separate ThreadStart/ThreadEnd events posting code in JVMTI VTMS transitions [v6]

[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 incrementally with one additional commit since the last revision:

  addressed review comment: add a couple of asserts

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13484/files
  - new: https://git.openjdk.org/jdk/pull/13484/files/debe49c3..157f33af

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13484&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13484&range=04-05

  Stats: 6 lines in 2 files changed: 2 ins; 0 del; 4 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