manager: Send pings in discoverManagedDevices

When devices go down unexpectedly, the a device server's local topology typically does not get wind of it. As the manager is also just getting a copy of the device server topology, it is not possible to infer the disappearance of a device from that. As only missing devices are restarted, this makes recovery difficult.

CTRL suggested to call slotPing on all devices assumed to be present, and remove them from the list accordingly.