text: mask some internal fields

Some fields from the stream packet context are not really useful to the
end user of the text output, so we mask them by default. They can be
displayed using the -v/--verbose flag. The masked fields are the same as
in previous version:
 - timestamp_begin
 - timestamp_end
 - events_discarded
 - content_size
 - packet_size
 - packet_seq_num

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