Allow retrying failed SFTP transfers

Currently, the REST API forbids retrying SFTP transfers because it might cause monitoring issues in some fringe cases where the transfer is made between 2 gateways.

However, in this case, the cons of this restriction far outweigh the cons, as resuming transfers is a common feature of SFTP. In consequence, the restriction on retries for SFTP should be removed, and the monitoring issues will have to be fixed later by extending the SFTP protocol.