RFR: 8371380: [LOOM] The debug agent should avoid enabling VIRTUAL_THREAD_START/END events when possible [v2]

[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 incrementally with one additional commit since the last revision:

  use EventIndex type

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28485/files
  - new: https://git.openjdk.org/jdk/pull/28485/files/e8b21dba..24010875

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28485&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28485&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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