Transfers: re-queueing complete download leads to aborted state

[JurgenR]

When re-queuing a complete download the transfer goes into an aborted state on the other clients. This is because the offset get calculated based on the existing file and thus the file connection closes almost instantly (this is still odd considering the library should wait for the uploader to close)

A quick fix would be to just delete the local file when re-queueing a complete transfer. However, it might be reasonable to make this an option.