Common: introduce new io code To avoid code duplication in accept, connect, reading and writing we add a new set of classes in `io` namespaces located in the following files: - stream.hpp, acceptor.hpp, connector.hpp These classes consist of pure abstract interfaces for I/O. Then we reimplement them in the following files: - socket_stream.hpp, socket_acceptor.hpp, socket_connector.hpp, - tls_stream.hpp, tls_acceptor.hpp, tls_conncetor.hpp (for SSL). This allows future independant connections such as DBus, fifo or any other fancy optional stuff. We also no longer need large class hierarchy such as `connection` for irccdctl controller or transport_server, transport_client classes.

2.1.0