HConnection interface is public but is used internally, and contains a bunch of methods

HConnection has too many methods for a public interface, and some of these should not be public.
It is used extensively for internal purposes, so we keep adding methods to it that may not make sense for public interface.

The idea is to create a separate internal interface inheriting HConnection, copy some methods to it and deprecate them on HConnection. New methods for internal use would be added to new interface; the deprecated methods would eventually be removed from public interface.