Add queue to client's operation dispatching (#356)

We run the entire exchange pipeline synchronously,
since that's predictable, which is how Wonka's
sources work.

When an exchange dispatches an operation, for
instance with reexecuteOperation, this operation
is hence run immediately, which can be confusing
and lead to state that disagrees with itself,
since the operation is essentially then "nested".

Instead what we want is to queue operations up
if they're dispatched nestedly, so from inside
an exchange, and flush this queue after the
original dispatch has completed.

src/client.ts

@@ -68,7 +68,24 @@ export class Client {
     // called to dispatch a new operation on the subject
     const [operations$, nextOperation] = makeSubject<Operation>();
     this.operations$ = operations$;
-    this.dispatchOperation = nextOperation;
+
+    // Internally operations aren't always dispatched immediately
+    // Since exchanges can dispatch and reexecute operations themselves,
+    // if we're inside an exchange we instead queue the operation and flush
+    // them in order after
+    const queuedOperations: Operation[] = [];
+    let isDispatching = false;
+
+    this.dispatchOperation = (operation: Operation) => {
+      queuedOperations.push(operation);
+      if (!isDispatching) {
+        isDispatching = true;
+        let queued;
+        while ((queued = queuedOperations.shift()) !== undefined)
+          nextOperation(queued);
+        isDispatching = false;
+      }
+    };
 
     const exchanges =
       opts.exchanges !== undefined ? opts.exchanges : defaultExchanges;