Re-add reentrancy avoidance

I removed the equivalency of this in facebook#22446. However, I didn't fully
understand the intended semantics of the spec but I understand this better.

I believe this fixes facebook#22772.

This includes the test from facebook#22889 but should ideally have one for Fizz.

Author: sebmarkbage

packages/react-server/src/ReactFizzServer.js

@@ -1950,6 +1950,10 @@ export function startFlowing(request: Request, destination: Destination): void {
   if (request.status === CLOSED) {
     return;
   }
+  if (request.destination !== null) {
+    // We're already flowing.
+    return;
+  }
   request.destination = destination;
   try {
     flushCompletedQueues(request, destination);

packages/react-server/src/ReactFlightServer.js

@@ -790,6 +790,10 @@ export function startFlowing(request: Request, destination: Destination): void {
   if (request.status === CLOSED) {
     return;
   }
+  if (request.destination !== null) {
+    // We're already flowing.
+    return;
+  }
   request.destination = destination;
   try {
     flushCompletedChunks(request, destination);