Review KCM (Kernel Connection Multiplexor) support

[tailhook]

KCM is a kernel facility that provides a message based interface over TCP. Links:

• http://kernelnewbies.org/Linux_4.6#head-d86a7a8affd7cefef85fff400e39403718bf647a
• https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/networking/kcm.txt?id=10016594f4c6b3ef34c5de97d8ab62205d9d26a5

With this in mind, it looks like we need to change how we're doing protocol parsing. For newer kernel (>4.6) we need to upload a BPF filter to do most of the work for us. If you're on older kernel we need to provide a fallback.

This may be done in two ways:

1. Make (use) a BPF interpreter in the userspace
2. Create a higher level abstraction/DSL, which either compiles to BPF and sent to kernel or interpreted directly

Either way, it looks like we want to take advantage of the KCM by default, instead of having two implementations for each protocol. We are already abstracting simple things here, like "wait for delimiter" and "wait for N bytes". The kernel facility is much more rich, and we should get advantage of the feature.

I'm not sure how well does it play with TLS. Presumably, we will be able to dissect TCP protocol packets by KCM and everything else only in userspace.

Update: It's likely a future of what rotor-stream should be, but probably we need to start another crate for this. Because it looks like semantics will be very different from what we have now.