Rename Transport/Application API

[jakobod]

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 ?