Rename Transport/Application API

We should think about using more general naming for the transport and application protocol APIs. The Application layer is using a message-oriented communication model while the transport layer is using all different types depending on the transport protocol in use.
Hence, it would be more clear to rename `write_packet` in the transport abstraction to something more general such as `send` with some clear documentation about the underlying model (`UDP`->packet, `SCTP`->message, `TCP`->stream).
The same goes for `handle_data` in the application abstraction, which could be renamed to something along the lines of `receive`.

Do you have an opinion on that @Neverlord @josephnoir ?