Isolate Postgres integration replication state

[danddanddand]

Summary

• use per-test Postgres replication slots for integration listeners and clean them up after stream shutdown
• surface async runStream/runSnapshot errors from test cleanup
• narrow reused snapshot subtests from broad *.* table selection to the tables created by each subtest

Testing

• git diff --check
• go test ./pkg/stream/integration
• make integration-test

Fixes #975

Note: the snapshot subtest table-narrowing hunks in snapshot_pg_integration_test.go are intentionally duplicated in #976 (they are needed there for strict-mode snapshots to pass CI). The hunks are identical, so the two PRs merge cleanly in either order.