Detect whether apt-get update has been run recently

Currently we don't run apt-get update more often than a certain time period
in landscape-client. However, we only track when the client initiated the apt-get
run, it could be something else on the system that also runs apt-get update.

We should try to detect whether someone else has run apt-get update recently,
and don't run it ourselves, if that's the case.

It should be possible to look at the files in /var/lib/apt/lists to see when they were
last changed. It's unclear whether we can detect an update that didn't contained
any new packages.