Rudder

With the changes only mode defined in #5293, we can imagine having nodes NEVER contacting Rudder because everything is in success.

From a use point of view, we need to have periodically some kind of "hey, I'm alive!" message, allowing to know that nobody actually shutted down the server.

There is 3 main parts on that feature:

- some UI bits allowing to configure how frequently a node send a heartbeat;
- some Rudder backend work to store that information by node and have the necessary plumbing to give the information to the corresponding node;
- on the node, the logic in techniques to understand and act accordingly to the configured heartbeat frequency.