- Keep using the same hyper client instead of building a new one for
each request
- This allows the client to reuse connections and improves the
performance of multiple requests by almost 2x.
- The playlist_tracks example takes 38 secs before and 20 secs after the
change to enumerate a 180 track playlist
- To avoid carrying the hyper Client generics through the whole project,
`ProxyConnector` is always used as the Connector, but disabled when
not using a proxy.
- The client creation is done lazily to keep the `HttpClient::new`
without a `Result` return type
