Scylla offsets should be loaded at different moment during task startup

[Bouncheck]

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.