Create a tracked "Session ID" and broadcast function to resolve 'multi-device state-races'

[Scottapotamas]

This is some kind of ephemeral volatile value the UI sends after successful handshake.
When the microcontroller resets, this goes to zero, and the UI can then know if the device state needs to be refreshed.

There needs to be a function for event-based "wants to change the sessionID" which is called by a different instance of the UI (think mobile+pc at once).

When the even comes in, the new sessionID needs to be sent to all interfaces (and then handling disconnected interfaces is a developer side output problem).

[Scottapotamas]

Questions that need to be resolved/cleared up:

• How much entropy is needed in the session ID value
• Does this work (untested)
• How well does this solve the issues

Probably needs electricui-interface#569 to be partially implemented to evaluate.