stream.c: allow stream PC's `events_discarded` field to be set by user

If a stream packet context `events_discarded` field is set a flush time,
check if its value keeps the count monotonic, and use this value to set
the stream's current discarded events count if so. If it's not set, do
like before and set it automatically from the stream's current count.

This will make it possible for a ctf.fs sink component to write the
original `events_discarded` field without having to read it, compute
the difference with the previous value, and use
bt_ctf_stream_append_discarded_events() to change the stream's discarded
events count.

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