Fix: race with the viewer and readiness of streams

Add a message to inform the relayd that all the streams of a certain
channels were sent so it can make them available to the viewer. This
fixes a race where the viewer could start reading some streams before
having received them all.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>