Helpful downloader patches

[dgarciabriseno]

• Change Start Time logic so the next iteration always uses the Oldest file that was picked up, but never goes earlier than the given start time CLI arg. This helps for image sources that have a less standard release cadence. e.g. if downloader starts, and the last file it downloads is April 25, then it will continuously scan that same time until it gets more data instead of moving the search window forward.