Metadata for instance_id and packet_seq_num

The UST tracer now stores a stream_instance_id in each packet to allow
the viewers to match streams split in multiple trace files.

Also, a sequence_number is stored in each packet so it is possible to
compute the number of lost packets in overwrite mode.

This is a locked-step with the corresponding lttng-ust commits.

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