trimmer: Update the bounds of the trimmed packets

trimmer: Update the bounds of the trimmed packets

Update the timestamp_begin field of the first packet of each stream and
the timestamp_end field of the last packet to reflect the fact that the
packet content got truncated by the user. This is important for the
packet intersection feature afterwards. This requires that we create new
packets and copy the events to assign them to the new packets.

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