datatransfer: fix incoming transfer

FTP/IncomingFileTransfer classes were not designed correctly
to talk between them.
This patch moves the file handling into IncomingFileTransfer class
and FtpServer only handles FTP protocol.

Change-Id: Ia12777dcb064b69b04bd5b6b3defc3ba3e84b2c3
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>