Drop stale unavailable pool connections

[Mirochill]

Summary

• remove an assigned connection from the pool if it still reports itself as available after raising ConnectionNotAvailable
• prevent HTTP/2 connections that have received GOAWAY from advertising availability
• add sync and async regression coverage for stale available connections

Closes #1066.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• Documentation is not updated because this is an internal connection-pool bug fix.

Validation

• Local tests were not run because this workspace is intentionally coordination-only and does not run third-party project test suites locally.
• Static validation only: git diff --check.
• Relying on GitHub Actions for remote validation.

[mbeijen]

There are multiple PRs for similar issues open in this repository; but there has not been much activity lately. In httpxyz, most of these issues have now been fixed and I've been upstreaming fixes (not these ones yet) to httpx2.