Introduce repository update periods

[AMDmi3]

Some repositories (crates.io, aur) are quite heavy to fetch, and fetching them also generates load on upstream. Since update duration is getting smaller and smaller with #956, we may need to introduce minimal update period for such repositories - this would

• generate less load on upstream
• make our updates even faster
• be useful in future when repositories are update independently

Plan:

• add repositories table field, last_update
• update it after successful fetch
• introduce repos.d property min_update_period
• check these and skip extra fetches

[AMDmi3]

The first idea was to also set default update period to 1 hour. However for now it leads to pessimization, causing database updates without any package changes. Setting default update period to 10mins, which practically disabled the limit as any database update cycle is longer than that. The correct solution would be to untie database updates from fetches (#551).