code: protected static HashMap<String, String> createInitialSubscribedStreamsToLastDiscoveredShardsState(List<String> streams) {
  		HashMap<String, String> initial = new HashMap<>();
  		for (String stream : streams) {
  			initial.put(stream, null);
  		}
  		return initial;
  	}
docs: Utility function to create an initial map of the last discovered shard id of each subscribed stream, set to null;
  This is called in the constructor; correct values will be set later on by calling advanceLastDiscoveredShardOfStream().
  
  @param streams the list of subscribed streams
  @return the initial map for subscribedStreamsToLastDiscoveredShardIds