return promise from updating autocomplete

so one can await if needed

src/components/views/rooms/BasicMessageComposer.js

@@ -304,7 +304,7 @@ export default class BasicMessageEditor extends React.Component {
         // not really, but we could not serialize the parts, and just change the autoCompleter
         partCreator.setAutoCompleteCreator(autoCompleteCreator(
             () => this._autocompleteRef,
-            query => this.setState({query}),
+            query => new Promise(resolve => this.setState({query}, resolve)),
         ));
         this.historyManager = new HistoryManager(partCreator);
         // initial render of model