Fix: get the stream_id when generating live beacons

When we send an empty index (beacon), we need to extract the stream_id
to avoid stalling the client on inactive streams on startup.
Since the live clients need to know this feature is implemented, we had
to bump the lttng-live protocol version.

This fix should be backported to stable-2.4 as well.

Refs: #811

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