Add Multithreaded Large File Upload

[sibblegp]

Add multithreading to file uploads. Check file size before uploading to prevent error.

[mitchellhuang]

I might take a stab at writing a PR for this. Which library should be used: multithreading, multiprocessing, or asyncio? My understanding of Python's GIL is that for socket IO operations like a download/upload we should be fine with threads.

Edit - Started working on it here: https://github.com/mitchellhuang/b2blaze/tree/upload-large-file