[INTERNAL|API] changed TransferModeListener interface

It was possible to obtain references to BytestreamConnections through a
method provided by this listener interface. As it should not be possible
to get those references the signature of the method is changed.

Also removed the useless clear method from the interface.

Change-Id: Ia9ac3019b244943928e7cbc9764d6669f34835c3
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1323
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>