gnosis / dex-services / 4754

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Health endpoint (#1304)

This PR introduces a `health` filter so that the price estimator can signal to our deployment that it is ready and we can implement rolling updates for 0 downtime goodness.

This is implemented in the `core` crate since `driver` will need to have an endpoint running in the background as well.

### Test Plan

New unit test for filter, and
```
$ cargo run -p price-estimator
```

While it is starting up, check that the `health` endpoint does not respond:
```
$ curl -i 'http://localhost:8080/health'
curl: (7) Failed to connect to localhost port 8080: Connection refused
```

Wait for the `price-estimator` to be ready:
```
2020-08-12T19:56:34.396Z INFO [price_estimator] Server ready.
```

Then check that the `health` endpoint responds correctly:
```
$ curl -i 'http://localhost:8080/health'
HTTP/1.1 204 No Content
cache-control: no-store
content-length: 0
date: Wed, 12 Aug 2020 19:56:36 GMT
```

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