Channel jamming

[s-tikhomirov]

I'm planning to submit an entry about channel jamming (in general, not only cycle jamming for PTLCs).

My main sources so far are:

• https://blog.bitmex.com/preventing-channel-jamming/
• https://github.com/t-bast/lightning-docs/blob/master/spam-prevention.md

and probably some research papers as well.

[LLFourn]

@s-tikhomirov excellent. I've had that as a TODO forever so I'm very keen to tick this one off.