Move receiving and sending procedures to up from TlsConnection and TcpConnection to Connection.
Manage gnutls_session_t with unique_ptr.
Implement smart pointers for GnuTLS types and use them in TlsConnection and TlsListener.
Simplify spart pointers for Gnutls types.
Put common TCP socket oriented code in separate class TcpConnectionSocket.
Put common TCP socket (server side) oriented code in separate class TcpListenerSocket.
Update Makefile and add TlsListener test.
Add missing tls_listener_test.cpp.
Implement TLS session handling for acepted descriptors in TlsConnection.
Add TlsConnection test.