Compute discarded events in live

We now save the indexes received in live in the two first elements of a
packet index. This allows us to compute the number of discarded events.

We only keep the packet index elements we need in live so we don't leak
packet indexes when reading a live trace over many days (or longer).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>