Cassandra: Use lower consistency level in `*_missing` methods when called from `*_add`

When allow_overwrite is False (the default), the Cassandra storage checks if objects exist before inserting them. This is best-effort, (because other workers may write between this check and the actual write anyway), so only meant as a performance optimization.

This check is performed with the same consistency level as other queries, ie. QUORUM in production.

However, since this is already "vulnerable" to TOCTOU issues, I think we might as well lower this level to 1. This lowers the load by sending queries to fewer nodes, and lowers latency of the query itself.

---

Migrated from T4643 (view on Phabricator)