Fix: wrong use of the relay_streams_sent in snapshot

The relay_streams_sent message is only useful in live sessions and
should only be sent after all the streams of a channel have been sent.

Here we were sending this message every time we sent a stream to the
relay during a snapshot which makes no sense.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>