GH-142513: Reimplement executor management

[markshannon]

Enforces the following two invariants:

• Invalidating an executor does not cause arbitrary code to run
• Executors are only freed at safe points

• Issue: Executor management code lacks proper invariants leading to bugs. #142513

[Fidget-Spinner]

Ok I've given this a quite thorough review. I thought about if it's possible for executors to be ressurected such that their refcnt != 0 anymore, but I don't see how that's possible considering executors are never reused.

[Fidget-Spinner]

Some very strange connection things happening with git at the moment.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit be95cb0 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F142931%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

[Fidget-Spinner]

@markshannon all the refleak buildbots don't run JIT precisely because the current refleak tests can't account for an executor being created in the midst of execution :)

If you want to get refleak + JIT working, you'd need to fix libregrtest

[markshannon]

The sole buildbot failure seems unrelated to this PR.