High-level caf-net primitive for custom binary protocols

[Neverlord]

At the moment, there is no high-level API in place for operating on the byte input of a socket. We have added flow-based file processing in #1573. Hence, the utilities for operating on sequences of bytes or characters are already in place.

If we implement an interface that basically turns a TCP connection into a flow, we get
#1408 for free, since we already have the other building blocks.