CDAP-805 Threadlocal datasets for workers

Now since only `datasetFramework.getDataset()` in method `getDataset(String name, Map<String, String> arguments)` is not thread-safe, and since it does not get called every time `getDataset` is called, we needn't make the whole `getDataset` method synchronized.  Only the call to `datasetFramework.getDataset` should be made synchronized.