exp/ssh: refactor halfConnection to transport
        This CL generalises the pair of halfConnection members that the
        serverConn holds into a single transport struct that is shared by
        both Server and Client, see also CL 5037047.

        This CL is a replacement for 5040046 which I closed by accident.

R=agl, bradfitz
CC=golang-dev
https://golang.org/cl/5075042