[FlexiDag] replace NodeHandle::get_dag function name with NodeHandle::blockDag

[nkysg]

We can see previous code

    pub fn txpool(&self) -> TxPoolService {
        self.registry
            .get_shared_sync::<TxPoolService>()
            .expect("TxPoolService must exist.")
    }

    pub fn get_dag(&self) -> Result<BlockDAG> {
        self.registry
            .get_shared_sync::<BlockDAG>()
            .map_err(|e| format_err!("Get BlockDAG error: {:?}", e))
    }

may be blockDag(&self) is better than get_dag