Fix: lttng-consumerd: cpu hotplug: send "streams_sent" command

When creating a new channel, the streams being sent to the relayd are
kept invisible to the live client until the "streams_sent" command is
received. This ensures the client does not see a partial stream set.

This "streams_sent" command needs to be sent on CPU hotplug too,
otherwise the live client handling within relayd is not aware of those
streams (they are never published).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>