Function parse_public_or_derived_idempotency_key_at 

pub fn parse_public_or_derived_idempotency_key_at(
    key: &str,
    now: i64,
) -> Result<i64, StatementRunError>

Like parse_public_idempotency_key_at, additionally admitting the server-derived script form (sql-script:<canonical-parent>:<index>).