[Specs] Add Pool protocol

[tiero]

This issue tracks ideas and discussions for the Pool concept.

Pool

_Providers can pool together liquidity registering their endpoints into a distributed service mesh, creating a direct bi-directional stream connection to communicate their reserves/prices. The pool acts also as a first responder for traders to lookup for the provider's aggregated market prices and to register the trader's limit order proposals.

The main purposes of Pool are:

1. Accept requests from traders using the same interface defined here.
2. Let Liquidity Providers register themself and keep track of the network topology.
3. Let the traders placing limit orders. Maybe dedicated protocol?

NOTICE The provider joining the pool MUST serves only trade requests coming to the the pool, cannot serve traders directly.