Account the lost packets in snapshot mode

When working in flight-recorder, we cannot rely on the absolute sequence
number because we only extract the content of the ring buffer
periodically. Moreover, we don't want to account the lost packets at the
beginning or the end of the snapshot since the "holes" don't appear in
the resulting snapshot, so we make sure we only account lost packets
located between extracted packets.

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