Scaling out the backend

[juliusknorr]

We need to investigate and be prepared to scale the whiteboard backend.

Luckily socket.io is well prepared for that with https://socket.io/docs/v4/using-multiple-nodes/

As Redis is a common architecture component with Nextcloud it probably makes the most sense to integrate https://socket.io/docs/v4/redis-streams-adapter/

• First introduce redis as an optional layer for storing room data
• Introduce redis adapter for socket.io to be able to scale the node process to multiple ones
• Load testing #107