DB models

A back-end of the DEX should be able to relay orders and keep them until the taker who wants to fill or partially fill the order appears. We need a model for orders which will be referred to an order book.
```
Order
makerAsset: String,
takerAsset: String,
amount: Bigint,
rate: Double,
makerAddress: String,
assetList: JSON
order: JSON
marketId: number
```
Except for the transaction field, other fields are only for querying transaction. These are also in the transaction.
Also, a model for the history of completed transactions is needed for future reference.
```
Deal
maker: String,
taker: String,
makerAsset: String,
takerAsset: String,
makerAmount: Integer,
takerAmount: Integer,
```

__DB have to be changed according to changed matching scheme__

Inizia esaminando lo schema di matching e i campi della transazione descritti nell’issue, quindi individua i modelli di persistenza esistenti del progetto e i punti di ingresso dell’order book. Definisci i modelli Order e Deal in base allo schema corrente, inclusi i campi di query necessari, e verifica che la struttura del database supporti il relaying, i fill parziali e la cronologia delle transazioni completate.

Scritto dal modello di indicizzazione a partire dal testo della issue.