Swarm swap accounting phase 1

[holisticode]

This PR introduces the first phase of swap accounting for swarm message exchange.

This current implementation only does the accounting part, no cryptoeconomic features are implemented yet (price negotiation, claim settling, etc.). This means this implementation just keeps track of how many units are exchanged between nodes and maintains a balance with them.

The currently only accounted protocol is swarm's streaming protocol (swarm/network/stream). The prices defined in that protocol are totally arbitrary and will require monitoring, simulations and analysis before they can be seriously taken into account.

Basic design:

p2p/protocols/accounting.go defines the basic interfaces:

Balance defines the actual accounting interface. It only has an Add function. This function will be called with a peer instance and an amount. If the amount is negative, then the local node is being debited, if it is positive, the local node is being credited.

Prices is an interface that defines how prices are being expected. Every accounted protocol should provide an implementation, and return a Price structure for every message for which it requires accounting. This struct contains a Value int64 field; negative means receiver pays, positive means the sender pays for the message.

An accounted protocol needs to provide a Hook inside its Spec, where it defines its messages. The Hook contains a Balance and a Prices interfaces, which allows the messaging infrastructure to transparently do the accounting.

[holisticode]

To introduce some metrics for this implementation, a new NewCounterForced as been added.
cc @fjl @nonsense

[holisticode]

Opened too early. Superseded by ethersphere/swarm#962