[EventDispatcher][Messenger] Add ScopedEventDispatcher, stop mutating the dispatcher of the application

A worker registers its stop listeners on the dispatcher it is given, and removes
them when the run ends. ScopedEventDispatcher gives those listeners the lifetime
of the run without touching the shared dispatcher: it holds them itself, copies
the listeners of the wrapped dispatcher over when one is added for the same
event so that the two sets interleave by priority, and hands an event no
listener was added for to the wrapped dispatcher.