Continuously flush proxy response

[benbjohnson]

Currently, the proxy server uses io.Copy() for the response which means the response is only flushed at the end. This won't work for streaming responses. Instead, it should manually implement a copy and call http.Flusher.Flush() after receiving each section of bytes.

Code: https://github.com/superfly/litefs/blob/117c6524594a5ecf0a126b56da7b1ed35090fc00/http/proxy_server.go#L325-L330

See also: https://community.fly.io/t/response-buffering-inconsistent-results-when-reading-from-http-with-text-event-stream-mime-type/8430/17

[kentcdodds]

This will be really nice to have fixed! Thank you 👏