Increasing WebSocket idle connections timeout

Hi, after 5 minutes of inactivity, WebSocket connections get disconnected. Is it possible to increase this timeout?

To reproduce:

1. Run a Docker container: 

   docker run --rm -p 8080:8080 apache/zeppelin:0.10.1

1. Open a WebSocket connection:

   wscat -c 'ws://localhost:8080/ws' 

1. Do nothing for 5 minutes.

This error appears:

Disconnected (code: 1001, reason: "java.util.concurrent.TimeoutException: Idle timeout expired: 300000/300000 ms")

The same behavior can be observed with the UI. If I stop working for more than 5 minutes on my notebook, but I keep it open in my browser, it refreshes and I lose my outputs.

Some kind of keepalive mechanism would actually be ideal, instead of increasing the timeout. What do you think?

Thank you.