Caching on disk of popular blocks

[michaelsbradleyjr]

In codex-storage/nim-codex#50 @Menduist made a point about a disk-backed cache for nodes which will download popular paying blocks, hoping to "resell"/redistribute them with a small premium, i.e. "CDN/cache nodes".

Some questions naturally follow:

• What cache policy would be best suited to that purpose, would LFU be preferable to LRU?
• How will CDN/cache nodes measure popularity of blocks? Measurements could be made re: want-have and want-block requests and data available in the DHT, but would it be something done continuously or on a timer? Any ideas on how we could make such analytics pluggable and entirely optional?