[WIP] Add updater command

[jfontan]

The updater wakes up periodically and searches for repositories to update. There is a configurable maximum of jobs per update cycle. Currently it has three searches that are done one after the other until the maximum number of jobs is reached or all the searches ar finished:

• Repositories updated x days ago (we call these "old"). The priority for these update jobs is high.
• Repositories updated x/2 days ago. The priority is normal.
• Any other repository ordered by the difference between the last commit time and the last update time. Stagnant repos will have longer difference between those two times. The priority is low.

Needs: #246 Related: #64