Function upsert_producer_seq 

pub fn upsert_producer_seq(
    tx: &Connection,
    producer: &str,
    seq: u64,
) -> Result<(), BookkeepingError>

Upsert the producer watermark inside the caller’s open transaction. MUST run in the same transaction as the effects it deduplicates, so a crash can never commit an effect without its receipt (design 306 §3.2).