Add database and storage monitoring

[ri0t]

We need a lightweight component (or two) that make sure, the archiving of data doesn't fall on our heads.

• [x] introduce monitoring component
• [x] get database storage sizes
• [x] log to systemlog
• [x] do regular checks
• [ ] ~establish an alert-pattern or configuration~
• [ ] ~monitor in realtime and~
• [ ] ~alert certain user roles~