Function maybe_purge_expired_recipient_addresses 

pub async fn maybe_purge_expired_recipient_addresses(
    conn: &mut PgConnection,
) -> ModelResult<u64>

Probabilistic purge instead of a cron, in the style of the token cleanups.

Only raw-address rows ever hold an address; a delivery addressed by user_id resolves the address from user_details at send time and stores nothing.