WebSockets: sticky sessions for UI server

[lorenzo-cavazzi]

Motivation

We have a mechanism to handle multiple user's WebSocket channels to prevent running the heartbeat functions more times than strictly required, but multiple tabs can actually connect to different instances of the UI server.

Proposal

We should have sticky sessions for the UI server. Renku-python/core already implemented this mechanism, we should sync with them and use a similar solution (if possible).

[lorenzo-cavazzi]

The UI server won't receive further improvements.