Transfers should resume from the beginning if no negociation is possible

Currently, when the gateway tries to resume an interrupted send transfer, if the protocol does not have a negotiation mechanism, the transfer is resumed at the last byte sent. This, however, can cause problems as there is no guarantee that all the bytes sent have been received. This may lead to incomplete files being sent without error.

The only solution is that, in the absence of a negotiation on where to resume, the data transfer should be resumed from the beginning. This might hurt performance on large files, but it is a better alternative than sending incomplete files. This solution mostly applies when the gateway is client, since most protocols don't allow the server to send data which the client did not ask for.