nwu-agent must be very lightweight and scalable.
We should be able to run it as often as possible, to improve administration freshness, but it must not affect system perfomance.
Besides considering using nwu-agent as an always running daemon (not useful at all for our current issue), nwu-agent could:
* Cache APT information and only do a "big run" if anything has changed
* Besides caching, keeping track of cache ctime is another option - simple but we need to consider all filesystems and fs options it could run in (although I don't know anything about a fs which can disable ctime).
