socket frontend: Manage open connections in listen_and_serve().

socket frontend: Manage open connections in listen_and_serve().

For that purpose, added a new object data type wrapping a connection. All open
connections are managed in a linked-link in the socket object. The main loop
now also waits data to be available on an open connection and then passed on
the connection object to the connection handler thread which is then supposed
to handle requests (not implemented yet).