Turn `Stand` into a factory of `EventDispatcher`s

[dmdashenkov]

There is no good way to know if an event is "dead" or not, as all events always have at least one dispatcher — Stand. We should make Stand register separate event dispatchers for each client subscription so that the number of dispatchers is EventBus reflects the use of events.

[dmdashenkov]

It is, however, dangerous in performance terms to create an instance of EventDispatcher per client subscription, as there may be thousands of the latter. It is worthwhile considering to create a distinct dispatcher per event type.