datatransfer: supports canceling while transfering and multi-tranfer

This patchs implies a lot of changes for file transfer.
First, a TURN connection is made only for one file transfer. This
allows us to cancel a transfer by closing this connection without
cancel future file transfers. The TURN connection is closed when a
transfer is finished or canceled. Avoid to keep unused connections.
Second point, a ftpserver uses the id of its attached stream.

Note: removed unimplemented refuse and abortStream in peer_connection

Change-Id: I65ba104066de0494ba5a7954f0fad63eb2c9d6ac
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>