re-renamed untyped_actor -> event_based_actor

with this patch, untyped_actor is called event_based_actor once more
and blocking_untyped_actor has been renamed to blocking_actor;
reason to this is, that untyped_actor is too general, as there
are several other classes that are indeed untyped actors but do not
inherit from event_based_actor (e.g. io::broker, actor_proxy)