Scylla offsets should be loaded at different moment during task startup

Right now loading offsets is coupled with `start(Map<String, String> settings)` responsible for loading configuration and other necessary modules. While that happens after `initialize​(SinkTaskContext context)` which takes care of initializing most of the context, the proper moment to set up offsets would be during call to `open(Collection)` which is not overridden at the moment.