🏎️ Speed up uploading by parallelizing uploads

[Jasper-Bekkers]

I don't think this is quite the right transposition of this code because it introduces some unwraps that weren't there before, however massively speeds up upload speeds by submitting all uploads in parallel and then joining on all of them, making it easy to hit full 1 Gbps speeds.

I think in the recursive upload case of large folders there are other optimizations that can be done to this routine (e.g. scan the filesystem first, then batch create all the folders on gdrive, then batch upload all files together) but I didn't quite get around to that.

For me this was one-off throw-away code tbh, so I'll likely won't be updating this PR, feel free to close if it's too much of a burden to take on these changes.