10932: Replaced the use of a list with a set to check if files on local collection...

10932: Replaced the use of a list with a set to check if files on local collection are on the local file list, so that the resume start time is greatly reduced.
Also, the save_state method was taking too much time on two operations: deepcopy() and json.dump(). Replaced both with just one call to json.dumps() that's a lot faster than json.dump().
This will improve overall performance on big file collections uploads.