[INTERNAL] make sure ByteStreamConnections are properly registered

The receiver of the BinaryChannelConnection was started to early.
Depending on the speed of the system it was possible that a connection
already dispatches data to the XMPPReceiver but was not yet registered
by the DataTransferManager itself.

Change-Id: Ice619e6eb1519d40679f63a765d9e4fdd8e1533a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/964
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>