data_transfer: do not restart download from zero if possible

The protocol already supports start/end but it was never used.
This patch:
+ Open the incoming file in append mode (else it will restart)
+ Pass start=detectedOffset to the peer to restart the download
where we need.
+ Only remove the file if the file is EOF or cancelled by the
user to avoid to restart from 0.

GitLab: #788
Change-Id: I8eee635a076e668af226fd9ad7ba372f6adae3f6