Run checksum check if -V and -c are used and file is completed

With -c option, aria2 can continue download after the existing file
position. If it is not completed, then after completion aria2 runs
checksum checking if available. But if existing file has already been
completed, then CreateRequestCommand exits without issuing checksum
checking. And aria2 treats it download error because it needs checksum
verification but it has not been done. This change fixes this by
properly checking download state and issue checksum checking before
CreateRequestCommand.