Parallellization for Uploads

[philsturgeon]

Currently uploads take a fairly long time and can block me doing other stuff. If I walk off the upload might fail (will be improved in #35), if I switch apps it will stop uploading, and when I sit there keeping an eye on it, the one at a time upload can be pretty slow even when I'm at the farm house or pub using their wifi.

To solve the "I've just popped back to the farmhouse for a cup of team and I'd like to upload 500 photos because im scared of losing the work" use case, I'd rather trash my battery trying to upload as much of this all as possible in as short amount of time. I am generally plugged in for upload anyway, farmhouse mains or battery pack.

Could we try and parallelize the upload so it's doing 3-6 trees at a time? I know this adds complexity to code, and possibly to the UI, but maybe it's not sooo bad, as anything that fails to upload will stay in the queue as it currently does?