Dispatch MatrixActions synchronously

Otherwise we risk blocking the dispatches on other work, and they
do not need to be done asynchronously.

This emerged as a bug where the room list appeared empty until
MatrixActions.sync dispatches all occured in one big lump, well
after the sync events being emitted by the js-sdk.

src/actions/MatrixActionCreators.js

@@ -101,7 +101,7 @@ export default {
      */
     _addMatrixClientListener(matrixClient, eventName, actionCreator) {
         const listener = (...args) => {
-            dis.dispatch(actionCreator(matrixClient, ...args));
+            dis.dispatch(actionCreator(matrixClient, ...args), true);
         };
         matrixClient.on(eventName, listener);
         this._matrixClientListenersStop.push(() => {