Minion disconnect event

Enables the minion to handle the situation where the connection to the master is lost. This is meant as a proof of concept and as a follow up to https://github.com/saltstack/salt/pull/13143.

Pull request 13143 enables the minions to choose its master from a list and jump to the next if the first one is full or not available while starting up.

This POC, if accepted, will be extended to enable the minion to jump to the next master while running.