Recover user sessions after Manager redeploy/crash

[martinstark]

By allowing the /heartbeat endpoint to re-create sessions, it would be possible to recover the list of participants in a line after a manager crash or redeploy. It would enable near seamless deploys or restarts while the app is actively in use.

Currently, the /heartbeat URL will respond with an error if the session is not found.

Instead, the manager should re-create the UserSession, which would then populate the Participant list for ongoing conferences. This could be achieved by using query parameters when the client makes the request:

/heartbeat?username=martin&productionId=1&lineId=2

The query parameters would only be parsed by the manager when a session is not found. If a session is not found, validate the query parameter values and create a new UserSession using the id from the incoming heartbeat.