Use a proper key-value store for the two SNS-related caches
This uses ReDB as a relatively simple pure-Rust key-value store to reimplement the SNS-related caches for auto-classify and search results.
Closes #205 (closed)
Still a draft as it needs an import tool which reads the current file-based auto-classify and bincode-based search results caches into the ReDB-based store.
Edited by Adam Reichold