Add an async upload queue

Async upload now spawns a thread for each file that is uploaded, which is a problem if youre uploading many large files at the same time. To prevent the plugin locking up due to too many threads, we should implement an upload queue with n worker threads that upload files. Something like https://testdriven.io/blog/developing-an-asynchronous-task-queue-in-python/

• add queue
• add tests to measure performance of async upload

┆Issue is synchronized with this Clickup task by Unito