Function deduplicate_dirty

Scan the collection of media items in `all`, grouping them by similarity and deduplicating each group with at least one `dirty` item in it, recording the results in the database.

(
    conn: &AsyncMutex<SqliteConnection>,
    image_locks: &LockMap<Arc<str>, ()>,
    image_dir: &str,
    cache_dir: &str,
    mut dirty: HashMap<String, Item>,
    all: Vec<Item>,
)