Improve upload_fileobj performance

[JohnHBrock]

Changing the file reader buffer from bytes to bytearray significantly reduces CPU usage. Using bytes is inefficient because it's immutable: you get the classic string building problem, where repeatedly appending to an immutable sequence requires O(n^2) operations.

From the python docs:

if concatenating bytes objects, you can similarly use bytes.join() or io.BytesIO, or you can do in-place concatenation with a bytearray object. bytearray objects are mutable and have an efficient overallocation mechanism

I tried io.BytesIO too, but bytearray has slightly better performance in my testing.

[terricain]

Looks good, makes total sense. Am away at kubecon at the moment but assuming the tests pass, I'll look at getting this merged when I'm back

[JohnHBrock]

Hi, just a reminder to review this when you get a chance. Thanks!

[terricain]

Sorry for the delay. Yep looks good