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travis-pro

separate HTTP server from metrics encoding (#1335)

This PR introduces a new HTTP server component for serving endpoints for service monitoring (such as prometheus metrics and eventually health endpoints).

### Test Plan

Rouille code was moved around. Added unit test for the HTTP router. Start the driver or price estimator and check the metrics route:
```
$ curl -i http://localhost:9586
HTTP/1.1 404 Not Found
Server: tiny-http (Rust)
Date: Tue, 18 Aug 2020 07:35:04 GMT
Content-Length: 0

$ curl -i http://localhost:9586/metrics
HTTP/1.1 200 OK
Server: tiny-http (Rust)
Date: Tue, 18 Aug 2020 07:35:08 GMT
Content-Type: text/plain; version=0.0.4
Content-Length: 13672

# HELP dfusion_service_http_latency Latency in seconds for HTTP request
# TYPE dfusion_service_http_latency histogram
dfusion_service_http_latency_bucket{request="dexag",le="0.005"} 0
...
```

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