implement observeChangesAsync on the client

packages/minimongo/cursor.js

@@ -373,6 +373,13 @@ export default class Cursor {
     return handle;
   }
 
+  observeChangesAsync(options) {
+    return new Promise((resolve) => {
+      const handle = this.observeChanges(options);
+      handle.isReadyPromise.then(() => resolve(handle));
+    });
+  }
+
   // XXX Maybe we need a version of observe that just calls a callback if
   // anything changed.
   _depend(changers, _allow_unordered) {

packages/mongo/mongo_driver.js

@@ -966,8 +966,7 @@ Cursor.prototype.observeChanges = function (callbacks, options = {}) {
 };
 
 Cursor.prototype.observeChangesAsync = async function (callbacks, options = {}) {
-  var self = this;
-  return self.observeChanges(callbacks, options);
+  return this.observeChanges(callbacks, options);
 };
 
 MongoConnection.prototype._createSynchronousCursor = function(