datatransfer: multi devices support when sending files

With the current implementation, when a user send a file to a contact
who got several devices connected, a request will be sent to all of
these devices and the first which answer will get the transfer. So,
a user can't know on which device they will get the file.

With this patch, in the same configuration the peer will receives
the file on all of its devices and add the possibility to accept
or refuse the file. To avoid to see multiple file transfers because
a peer has several devices, I introduced a OptimisticMetaOutgoingInfo
which represents the best current state of the transfer (for example,
if one subtransfer is ongoing and one failed (cause refused) it will
show the outgoing one).

This is an ongoing work, in the near future we must:
+ Give the ability to clients to see the real status of each
subtransfer (with the ability to cancel/retry transfer by devices)
+ Add a timer to close awaiting transfers to avoid to let a unused
connection for too long.

Change-Id: I84eb243bff2bfdc087a83dd7eced45c361f27d16
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>