Std net.Conn already can represent both plain TCP connections and
connections wrapped with TLS. However the connections itself need to be
created differently. This might become inconvenient when establishing
connections should be inside server logic where it is desirable to have
one codepath which works uniformly via interfaces.

This patch introduces Networker - new interface which represents
access-point to a streaming network. One can Dial or Listen on it and
get underlying network name.

Two functions are also provided to create networkers for plain TCP
and to wrap a networker with TLS layer.

This way one can initially decide and setup a networker, pass it to
server logic, and server inside uses just Networker interface
transparently either listening/connecting via regular sockets, or via
sockets wrapped with TLS layer.