Provide HTTP Keep Alive for Web and Web Sockets

In environments that utilize TCP Proxies for connections. (Common in Mesos Environments using HAProxy and others). A situation occurs that connections to the GUI and Web Sockets keep TCP sessions open with no data. This makes the TCP Proxies timeout, considering no data a dead connection. This CAN be tuned on the proxy, but it's a bandaid as it's hard to set a value one notebooks that WILL indicate a truly lost connection vs a Notebook that is open with no activity.

What happens in the notebook, is after no activity in the TCP Proxy Timeout, the notebooks goes to a "Disconnected" state and the page must be refreshed before anything can be run. This is frustrating and may lead to edge case situations were work done by a user is altered in unexpected ways.

To address this, I propose allowing an option for the web gui and web socks to have a configurable HTTP Keep Alive value in which if there is no user activity, and the notebook is still open, that a keep alive is sent to keep the TCP Active. This would allow flexibility and provide for a better user expierience.