athenad: fix thread safety issues in upload handing

[deanlee]

This PR addresses thread safety issues by adding synchronization for shared variables, which previously led to race conditions and data inconsistencies. The updates enhance data integrity and system stability.

1. Simplified Queue Management: Removed cancelled_uploads. Canceled items are now directly removed from upload_queue in the cancelUpload function, reducing complexity and ensuring consistent synchronization.
2. Thread-Safe Access: Direct access to upload_queue.queue bypassed the Queue’s synchronization. This update ensures thread-safe access by using the mutex lock provided by the Queue, preventing race conditions.
3. Mutex Protection for cur_upload_items: Ensured safe access with a mutex for cur_upload_items.