gnosis / dex-services / 5103

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travis-pro

consistent dust trade handling (#1401)

Fixes #1355

This PR adds dust amount checks for the estimated prices and orders. This was previously ignored and estimated amounts were returned for dust trades when they are not actually matchable.

### Test Plan

Added unit tests and make sure that the GUSD routes work as expected:

<details><summary>Buy amount in base is larger than dust amount</summary>

```
$ curl -s 'http://localhost:8080/api/v1/markets/6-2/estimated-buy-amount/103.19' | jq
{
  "baseTokenId": 6,
  "quoteTokenId": 2,
  "buyAmountInBase": "100",
  "sellAmountInQuote": "103.19"
}
$ curl -s 'http://localhost:8080/api/v1/markets/6-2/estimated-buy-amount/103.18' | jq
{
  "baseTokenId": 6,
  "quoteTokenId": 2,
  "buyAmountInBase": "0",
  "sellAmountInQuote": "103.18"
}
```

</details>

<details><summary>Sell amount <strong>after rounding buffer</strong> is larger than dust amount</summary>

```
$ curl -s 'http://localhost:8080/api/v1/markets/2-6/estimated-buy-amount/101' | jq   
{
  "baseTokenId": 2,
  "quoteTokenId": 6,
  "buyAmountInBase": "100.787875",
  "sellAmountInQuote": "101"
}
$ curl -s 'http://localhost:8080/api/v1/markets/2-6/estimated-buy-amount/100' | jq
{
  "baseTokenId": 2,
  "quoteTokenId": 6,
  "buyAmountInBase": "0",
  "sellAmountInQuote": "100"
}
$ curl -s 'http://localhost:8080/api/v1/markets/2-6/estimated-buy-amount/99' | jq 
{
  "baseTokenId": 2,
  "quoteTokenId": 6,
  "buyAmountInBase": "0",
  "sellAmountInQuote": "99"
}
```

</details>

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