server: hosts are only probed once at start

[SpComb]

The will only probe each host once when loading a new host. The probing determines the supported host MIBs and the Online state, so if the probe fails, then the offline host will not be included in any /api/objects/... queries, and the empty mibs will result in an empty /api/hosts/.../objects/ query set. Conversely, if the probing succeeds but the host later goes offline, then any /api/objects/... queries will get delayed waiting for that host to timeout.

The host online states should be polled and updated at runtime to avoid those problems.

Workaround: Use PUT /api/hosts/... to re-load the host.

Going a bit further for the long term, the host mibs might also need to get re-probed if the device is upgraded... The hostname might also need to get re-resolved if the device moves? Alternatively, just require the server to be restarted in those cases...