exp/ticker: create an orderbook scraper to compute stats

[accordeiro]

Create a script that scrapes orderbooks to compute spreads and other stats.

Tasks:

• Adapt db models
• Create scripts
• Write tests
• Adapt the market data aggregator to also include the stats
• Adapt the JSON exporter to also provide the computed stats on markets.json
• Add GraphQL support
• Add cron job to refresh orderbook stats

[accordeiro]

Closing this, as the functionality would be identical to what is already provided by the Horizon Orderbook Details Endpoint [CC @tomquisel].

[tomquisel]

@accordeiro I think the idea of this scraper is to add orderbook stats to the ticker, so a site can quickly look at the spreads on all markets without making many requests to the Horizon orderbook endpoint.

In the StellarX GraphQL schema, the OrderBookStats struct is included for each market:

type OrderbookStats {
  nBids: Int!
  bidVolume: String!
  bidMax: String!
  nAsks: Int!
  askVolume: String!
  askMin: String!
  spread: String
  spreadMidPoint: String
}

I think you can provide all this information even in markets.json, but you'd have to be clever about aggregating across markets since bidMax and askMin could potentially cross.