do a sanity check when conduction upgrades

[Denis-de-Bernardy]

In the FTP transports, and possibly in the SSH transport, we should look into conducting a quick sanity check after WP transfers a file during upgrades. This would be in the event that a network problem occurs during the upload.

Depending on how robust we want it, it could be as simplistic as checking the file's size, or as complex as using md5_file().