[RFE] Cache tiering

[tonobo]

Due to limited amount of memory we don't want to keep all the snippets all the time at redis. But to keep low latency were must using redis. So the idea is to implement a cache tiering. The api should be able to subscribe on redis keyspace notifications. If an event was raised, the api should delete the key from redis and push it to a persistent layer.

But currently i haven't got a solution for handling missing objects. Due to requesting missing objects were got the problem to call all caching tiers, so we're need there a ratelimit which handles this known fuckup.

[tonobo]

Another idea is to just expire the snippets instead of full gists. Feching snippets direktly is also not allowed at the moment. By fetching snippets from gist we will implement the cache tiering layer.