Fix: lttng-live discarded event count after inactivity

When a stream is inactive, the consumer produces fake indexes which are
beacons to let the viewer know a stream has not produced any data up to
a certain timestamp. These beacon are actually real packet indexes with
all the fields set to 0 except for the timestamp_end. Currently we keep
these beacons just like we keep real indexes. The problem is that when
we switch packet, we compare the events_discarded field in the index we
just received with the same field in the previous index. In the case
where a stream has been inactive, we have received inactivity beacons,
and set the discarded_event field to 0, so the difference with the next
real index might be wrong.

In fact, since the inactivity beacons are only used to push the
timestamp end of a stream, we don't need to keep them and we actually
need to keep most of the data from the real previous index. So we now
copy the entire prev_index into the cur_index when we receive an
inactivity beacon. We could refactor the code to avoid performing the
pointer swap of cur and prev indexes, but this implies a redesign of
much of the packet switching code which would affect other code paths.

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