[EventLoop] Associate only one listener per each stream event type.
[EventLoop] Associate only one listener per each stream event type.

Allowing multiple listeners makes code more complex and it is beyond
the scope of the whole event loop abstraction anyway.

Such a requirement could be easily satisfied on a higher level if a
class wrapping the event loop needs to notify more than one listener
about changes in the status of a stream resource.

steverhoades/event-loop,kaduev13/event-loop,mkrauser/event-loop,kaduev13/event-loop,reactphp/event-loop,steverhoades/event-loop,clue-labs/event-loop,mkrauser/event-loop