Track down in memory only when a switch/link/interface status has changed

[viniarck]

Track down in memory only when a switch/link/interface status has changed

This is something that @RenataFrez has highlighted as being useful.

This is still needs discussion but I'll capture here, the idea is something like status_changed_at: datetime.

Certain status change might be too frequent, so this shouldn't be stored in the database and this field isn't critical, so during controller restarts resetting this should be OK.