Investigate database buffered counters for cached responses

From !157055 (comment 2000995277).

The cached response downloads_count column could be a test bed for Project statistics: introduce database buffered... (#438565).

• Explore how database buffered counters could look like.
• Explore the effort to implement it for cached responses downloads_count.
• While doing so, think about a generic enough solution so that this can be applied to other columns.
  • Use concerns/modules and/or parent classes so that developper's work is reduced to: hey, on this model, this is a databased buffered counter.