RFR: 8371380: [LOOM] The debug agent should avoid enabling VIRTUAL_THREAD_START/END events when possible [v3]

[Chris Plummer]

> Don't enabled VIRTUAL_THREAD_START/END events unless absolutely necessary. Solves performance issues when trying to debug apps that create a lot of virtual threads. Details in first comment.
> 
> With these changes the Skynet benchmark no longer shows any slowdown when launching with debugging enabled or when attaching the debugger.
> 
> Tested with all tier2, tier3, tier5, and tier6 CI testing (with filters to only run svc tests).

Chris Plummer has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge
 - use EventIndex type
 - fix whitespace
 - disable VIRTUAL_THREAD_START adn VIRTUAL_THREAD_END unless needed

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Changes: https://git.openjdk.org/jdk/pull/28485/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28485&range=02
  Stats: 312 lines in 7 files changed: 253 ins; 36 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/28485.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28485/head:pull/28485

PR: https://git.openjdk.org/jdk/pull/28485