Implement cheque exchange protocol

[holisticode]

For settlement of imbalances, the SW3 paper at https://www.overleaf.com/project/59c298da5b3a6e3b7ec80d56 proposes a Swarm-integrated protocol with an exchange of cheques. Refer to chapter 2.2 of that document for a description.

This issue is about defining and implementing the cheque exchange protocol in the go node code.

It will have to define and implement:

• The Swap cheque data structure
• The protocol message format and data structures
• The message handlers for these messages

All these steps need to also be properly documented (swarm-guide).