RFR: 8306028: separate ThreadStart/ThreadEnd events posting code in JVMTI VTMS transitions [v5]

[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 two additional commits since the last revision:

 - Merge branch 'br29' of https://github.com/sspitsyn/jdk into br29
   merge with branch29
 - move code a little bit

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13484/files
  - new: https://git.openjdk.org/jdk/pull/13484/files/639b2110..debe49c3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13484&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13484&range=03-04

  Stats: 52 lines in 2 files changed: 24 ins; 24 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