hide HandleRequest method from connection interface

[ksysoev]

We can simplify connection interface by refactoring some current logic

currently we block with handling request inside channel abstraction, but if we move this processing inside connection registry, we'll be able to hide HandleRequests from public.
Other benefit that we'll be able to hand on close signal directly in connection registry. without onClose channel, and remove such responsibility from connection abstraction.

[ksysoev]

implemented in #65