Avoid crash when a running SqlQueryMaker is deleted

Deletion only after a job has finished seems to prevent crashes in cases where an already deleted job finishes and runs defaultEnd, and additionally cases where SqlQueryMakerInternal is deleted during an sql query and it tries to emit now deleted signals based on query results.

Fixes a flaky test, hopefully fixes also fixes a related bug.

BUG: 431738

When I started inspecting this, I had about 10 % idea what I was doing. After digging and trying out a hundred different things, I guess I understood and learned various things in process and I'm standing at a solid 50 % understanding now. The explanation makes sense, a test got fixed, I didn't manage to reproduce the bug earlier but at least based on quick testing, nothing seems to have been broken by this in real usage, either.