Fix: stream.c: autopopulate timestamp fields even when not mapped to CC

To remain backward compatible with Babeltrace 1's CTF writer,
automatically set the event header `timestamp` fields (when appending
the event) and packet context `timestamp_begin` and `timestamp_end`
fields (on stream flush) when the stream's class has a registered CTF
writer clock, even if the field's types are not mapped to a clock class.

If the field is mapped to a clock class, then it is automatically set
when this mapped clock class is the same as the stream's class's clock's
class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>