Make connection handler deal with all addressing, and hide the internals
of security handler.
Will allow security handler code to be stubbed out if mbed TLS is not
available.
When receiving data, destination (own) address is stored and given to
socket when sending reply to make sure respone comes always from same
address where request was sent. This commit supports only real socket to
real socket messages, not virtual socket, or DTLS handshake messages.