Negotiate on chunk price in link

[gbalint]

Upon connection nodes should agree on a swap chunk price
Minimal implementation: offer prices, if it is the same, connect, otherwise do not connect.
Use a default price

[holisticode]

I wonder though - that should result in different prices between different nodes? Is that a problem at all?

[holisticode]

I also believe there should be a basic documentation explaining how the swap accounting should work (e.g. what is written in this issue, that is will be accounted in ETH, ...)