src.ctf.fs: use one CTF notif iter per notification iterator

src.ctf.fs: use one CTF notif iter per notification iterator

Use only one bt_ctf_notif_iter per src.ctf.fs notification iterator,
instead of one per data stream file, so that this object can keep a
state for the whole packet sequence.

ctf_fs_ds_file_create() now receives the notif iter to use and sets
itself as its current medops user data.

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