refactored mem mgmnt in MM

this patch changes the memory management in the middleman, i.e.,
´continuable´ no longer reference counted but provides a `dispose`
member function; using reference counting to manage object lifetimes
in the MM is an overkill, because it is quite simple to decide when
an IO worker is done; this change allowed us to remove `broker_backend`,
because `broker` can now inherit both `local_actor` and `continuable`
without having two reference counts