More checks for folder upload and download

[robinrodricks]

Currently when uploading/downloading a folder, you can only choose to transfer files that don't already exist, ie. performing a file exists check. You cannot re-transfer a file if the size mismatches or it has changed. So we need more checks:

1. File size is different
2. File CRC mismatches - use CRC if supported for fast file contents comparison
3. File date modified is different

[robinrodricks]

Added to the bucket list. We will pick this up as and when we have free time. Comment on this issue if you want us to prioritize it. Thanks!