[#780] make sure to dispatch requests in connection buffer

This should completely fix #780 irrespective of what we do with #750. 

Let's remove `nrepl--make-default-handler` and throw an error instead.  All requests must have callbacks. If we get to `nrepl--make-default-handler` then we are doing something wrong.  If I am not mistaken you intended to get rid of it in the past.