Move the sessions polling to the server and notify using the websocket channel

[lorenzo-cavazzi]

Motivation

As a follow-up to #1338, we should stop looping for the sessions in the UI client and move the logic to the UI server.

Plan

This is broadly how this should work:

SERVER

• Query sessions in the short loop, hash the whole content, and store it locally (we might want to remove data we expect to change often like resources usage and timestamps, or we might even just take a slice of the whole response as far as it detects all the relevant changes)
• Compare the hashed content to verify whether something changed. If that's the case, store the new content and send a "sessions changed" notification to the client.

CLIENT

• When getting a "sessions changed" notification, refretch the sessions when relevant . This might either be always or depending on the current page (i.e. relevant when look at any project page or any session page).

INFO
A stub of the code is already available here https://github.com/SwissDataScienceCenter/renku-ui/blob/master/server/src/websocket/handlers/sessions.ts
Please mind that this was made for a previous POC and the logic has changed a bit in the meanwhile (E.G. we might not want "session-id" or "storage" anymore, we can cache on the channel data property; we also want to notify all the channels directly from the function).

[lorenzo-cavazzi]

We should start with a first POC using the sessions notifications to re-fetch content only when we are not in a session page.

• The client asks for sessions at startup
• The server keeps sending information
• The client refetch sessions unless:
  • we are visualizing a session page
  • sessions are being fetched or were already refreshed lass than 3 seconds ago.