Function deduplicate

Query the database for items for which we have not yet calculated a perceptual hash (e.g. files that have been newly added), calculate their p-hashes, and look for duplicates among all files which have similar p-hashes.

(
    conn: &AsyncMutex<SqliteConnection>,
    image_locks: &LockMap<Arc<str>, ()>,
    image_dir: &str,
    cache_dir: &str,
)