Keep-alive in /pods upload

[sharafian]

Codiusd doesn't have a timeout on incoming HTTP requests, but lots of them are timing out when the payment size is large or it takes a long time to pull images. This is because nginx returns a 504 when the upstream call takes too long to send data.

We could prevent this by having codiusd send a small amount of data on an interval in order to keep the connection alive. We could use the raw response stream in order to write some insignificant data (like whitespace) and then write the actual JSON response once the request is complete