Fixed support for multi-subscribers

[bok-]

📒 Description

With Vexil 3 we were relying on AsyncStream's unofficial handling of multiple subscribers/iterators. While it did work there were known bugs with cleanup of multi-iterators, and it was considered an accident that it supported before - it was not behaviour that was guaranteed.

Now that we've had more evidence of Vexil 3 used in production projects it is apparent that relying on AsyncStream's behaviour here leads to unreliable outcomes, so we need a custom solution.

This PR adds that, and modernises our locking infrastructure with a more modern backport.

🗳 Test Plan

Covered by unit tests.

🧯 Source Impact

Publicly this changes only one API — when subscribing to changes it now returns a custom AsyncSequence instead of an AsyncStream.

✅ Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud