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

[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.

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Commit messages:
 - 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=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306028
  Stats: 161 lines in 4 files changed: 86 ins; 61 del; 14 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