Heartbeat functionality is wanted

[hmpf]

Other parts of geant than the noc are looking for monitoring if devices stop sending data. There have also been internal requests for a built-in heartbeat functionality (see #668).

Idea:

• mark which sources sends heartbeats and how often (schema change, API call!)
• send a stateless incident (perhaps with a new event type HEARTBEAT?), level 5
• have argus make a stateful event if a heartbeat-sending source hasn't sent at heartbeat within the time period stored on the source

This will have to wait until after we have a task-queue.

It is possible to have an agent to monitor incidents but then that works as a glue service and needs a heartbeat of its own.