Support Rclone concurrency

[cyberpower678]

Just a fair warning, I actually haven't tested this code due to lack of time, but it SHOULD in theory work as expected. Setting RCLONE_TRANSFERS will trigger the concurrency, and it will default to 1 if absent.

I also fixed a possible bug where a failed upload might drop from the missing upload event IDs list because the current_event variable may not get reset to None if something goes wrong during upload.