Make caching a config option

[ckcr4lyf]

Currently we cache announces in redis as a key, so we can GET it instead of ZRANGE on seeders & leechers, to imporove performance.

Of course, nothing is free, and for us this results in larger memory usage by redis (for caching of course).

While kiryuu is targeting performance, we should not assume the compromises that a user is willing to make (CPU vs RAM), depends on their available resources.

As such, we should allow deciding whether to enable the cache or not using a config option (compile time?)