Fix flaky test tests/test_db_async.py::test_two_concurrent_requests_each_hold_their_own_connection_only_during_their_statements

The merge lane saw `tests/test_db_async.py::test_two_concurrent_requests_each_hold_their_own_connection_only_during_their_statements` fail and then pass on a rerun of the same tree. - Test: `tests/test_db_async.py::test_two_concurrent_requests_each_hold_their_own_connection_only_during_their_statements` - Tree: `9b3468f490401c060461dc777263986aaafd6b65` - Lane task: 13104 - Flake record: 01a1a004-af7e-7151-b4dd-f5746542ee33 Make it deterministic; do not delete it unless it tests nothing.

The flaky concurrent-connection test now uses a barrier rendezvous instead of a timer-sampled pg_sleep overlap, keeping all its assertions. 12 runs on a loaded host passed and the quick gate is green. PR #7832 is queued to the merge lane. Memory: used mem-25288.