Fix: lttng-sessiond: cpu hotplug: send channel to consumer only once

Fix: lttng-sessiond: cpu hotplug: send channel to consumer only once

On CPU hotplug, we currently send a duplicate of the channel key, which
allocates its own object (duplicated) within the consumerd. We want the
newly added stream to map to the pre-existing channel key, so don't send
the channel duplicate.

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