[ExtJS] Control Ext.data.Batch start for multiple stores serialization

gistfile1.js

var batch = new Ext.data.Batch();
batch.start = function () {};
// Sync your stores
myStore.sync({
  batch: batch
});
// When done
delete batch.start;
batch.start();

By setting start at the object level, you override the prototype member and make sure that the batch won't start until you want it to start.
Once done, deleting the object member restores the prototype one visibility.