Implement keepalive message in socketio adapter

[vincentfretin]

You may think the socketio adapter is stable/reliable when just using the node server, this may be true, but it is not the case when you use it behind nginx, see #200
The socketio adapter (and also the webrtc adapter) in this repository lacks any sort of keepalive message, so if there is no component updates going through the socket for 30s (it depends on the keepalive_timeout param in nginx), the WebSocket will just close.

Please use the easyrtc/wseasyrtc adapters with open-easyrtc for production behind nginx, open-easyrtc is sending a stillAlive message every 20s by default.

I'm open to any PR that implement a keepalive message like it's done in open-easyrtc or janus adapter via the minijanus library if you want inspiration. I won't implement it myself though.

[vincentfretin]

You can also implement something yourself in you app to send regularly a component update like periodic-full-syncs component or your avatar, see https://github.com/mozilla/hubs/blob/5fc48ef25336057ef7fb985eac5b2fcb3937f567/src/components/periodic-full-syncs.js