Add CacheSQLite (#13131)

Persist is intended for larger objects, which works best for things like cached tiles with some metadata.

For many small to medium entries that you nevertheless don't necessarily want to keep around in memory, that seems a bit inefficient.

SQLite then, is effectively a way of minimizing the amount of writes in this scenario.

Relates to #13061. Depends on #13122.

frontend/cache.lua

@@ -1,6 +1,8 @@
---[[
-A LRU cache, based on https://github.com/starius/lua-lru
-]]--
+--[[--
+An LRU cache, based on https://github.com/starius/lua-lru
+
+@module cache
+--]]
 
 local lfs = require("libs/libkoreader-lfs")
 local logger = require("logger")