code: function _lookupByNodeIdentifier(nodeIdentifier, timeoutMs) {
          // if the address is cached, return that
          if (cachedNodes[nodeIdentifier]) {
              return rx.of({ destination64: cachedNodes[nodeIdentifier]});
          }
  
          if (debug) {
              console.log("Looking up", nodeIdentifier);
          }
  
          return _localCommand("DN", timeoutMs, nodeIdentifier).pipe(
              rx.operators.catchError(function() {
                  return rx.throwError(new Error("Node not found"));
              }),
              rx.operators.map(_extractDestination64),
              rx.operators.tap(function(address64) {
                  cachedNodes[nodeIdentifier] = address64;
              }),
              rx.operators.map(function(address64) {
                  return { destination64: address64 };
              })
          );
      }
docs: Returns a stream that will emit the 64 bit address of the node with the given node identifier.