Redirect callback to dummy methods after unsubscription

Inspired by #672 (closed),

Related to #185 (closed), #371, !415 (closed)

Our current mechanism of keeping reference to callback after unsubscribe_event is a bit ugly: callback release happens only after user called unsubscribe_event one more time minimum one minute after.

I would propose a simple solution - we redirect push_event to the dummy method, which we can keep referenced as log as we need and release user method immediately