Add explicity for EventStream

 * show it explicity by NotImplementedError that EventStream is abstract class
 * add comments to describe difference between ArrayEventStream and MultiEventStream
 * remove comment-outed methods of ArrayEventStream to break interoperability of EventStream
 * remove comment-outed IoEventStream:
   * that is not implemented and is not going to be implemented
   * that is danger because it require io-stream to be opened until the EventStream is destructed