Statuses: Timestamps & Messages

[aluzzardi]

We're fixing task statuses (#323) to include timestamps and messages.

We should do the same for any other statuses, for instance, it would be valuable to know for how long a node has been down, when a certificate was issued, etc.

/cc @stevvooe @diogomonica

[aluzzardi]

@diogomonica @aaronlehmann @stevvooe

I think this is still relevant: we should add a timestamp to NodeStatus, UpdateStatus, ...

[diogomonica]

Sounds good.