Convert the CacheManager class to use SQLite

[doccaz]

Due to the potential amount of small writes with multiple instances, the current cache implementation (plain JSON file) only reads the cache at the startup, modifies it in-memory, then writes it at the end. This is not optimal, and as a collateral effect "only the last one wins". With an SQLite backend, I hope to mitigate this.

Curiously, the current implementation has the thread locking working fine and there have been no data corruptions, but SQLite would also help with that too.