Start first incremental sync request early (#629)

* Start first incremental sync request early

So it can run while we process our sync data.

src/store/indexeddb-local-backend.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2017 Vector Creations Ltd
+Copyright 2018 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -243,6 +244,10 @@ LocalIndexedDBStoreBackend.prototype = {
         }
     },
 
+    getNextBatchToken: function() {
+        return Promise.resolve(this._syncAccumulator.getNextBatchToken());
+    },
+
     setSyncData: function(syncData) {
         return Promise.resolve().then(() => {
             this._syncAccumulator.accumulate(syncData);