fn execute_job<Qcx, K, V>(
    qcx: Qcx,
    key: K,
    dep_node_opt: Option<DepNode<Qcx::DepKind>>,
    query: &QueryVTable<Qcx, K, V>,
    job_id: QueryJobId
) -> (V, DepNodeIndex)where
    K: Clone + DepNodeParams<Qcx::DepContext>,
    V: Debug,
    Qcx: QueryContext,