ctf: notif-iter: ensure that all packets refer to the same stream class

ctf: notif-iter: ensure that all packets refer to the same stream class

Disallow, within the same packet sequence (decoded by a single CTF notif
iter instance), two packets to be described by two different stream
classes, which would also mean two different streams. Log a specific
warning message when this happens, e.g.:

    07-05 12:05:04.235 17923 17923 W PLUGIN-CTF-NOTIF-ITER
    set_current_stream_class@notif-iter.c:961 Two packets refer to two
    different stream classes within the same packet sequence:
    notit-addr=0xd97b20, prev-stream-class-addr=0xdc0b20,
    prev-stream-class-name="(null)", prev-stream-class-id=33,
    next-stream-class-addr=0xdc13d0, next-stream-class-name="(null)",
    next-stream-class-id=34, trace-addr=0xda3650, trace-name="ctf2"

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