[bisq-network/proposals] Market price feed aggregator (#26)

[Manfred Karrer]

## Overview
We use BitcoinAverage for Fiat market prices and Poloniex and CoinMarketCap for altcoin market prices. The price data are requested by the price-nodes from those services and Bisq nodes are requesting via http over Tor the price data from our price-nodes.

## Problem
We have an open vulnerability for single point of failure in case BitcoinAverage would fail (happens once in the past) as most Bisq offers are using percentage based prices and therefore are dependent on price data. 
To a lower extent that is the same for altcoin prices but as we have 2 service providers (Poloniex and CoinMarketCap) it has already a bit more resilience.
Furthermore it comes with a security risk as in case the price provider would deliver wrong data, price based Bisq offers would be taken with that wrong price.

## Goal
We would like to have more resilience for those price feeds, so in case one of the services fail or deliver incorrect data the Bisq users are not affected.
It is not intended to offer that service outside of the context of Bisq.

## Proposal
We should implement basically our own lightweight version of BitcoinAverage. Collect price data from the top 5 exchanges and apply a similar weighting algorithm as used in BitcoinAverage on each of the sources to get an average price. 
Use national currency exchange rates to convert it to other currencies. 
We should try to get in that way a similar result to what BitcoinAverage delivers and then we could mix both services to get the price for Bisq users. If one of the sources deliver very different prices it should be filtered out and the maintainers should get alerted to investigate the issue.
For altcoins we could do something similar or find 1 or 2 other price providers so we get more resilience.  

## Implementation
The service could be added to the price-nodes or run as separate service and the price-nodes consumes that service (could be on the same host machine). 
It does not require special Bisq or P2P network knowledge so it should be easier to find a developer who can work on that. 

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https://github.com/bisq-network/proposals/issues/26
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