ctf: msg-iter.c: rename `notit` to `msg_it`

Messages used to be called notifications. The CTF message iterator used
to be called notification iterator. This explains why the msg-iter.c
file has tons of variables and parameters named `notit`.

I believe it's time to rename them to a proper name: `msg_it`.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If546b53a9bca2495fb1cfaf0232d528478e659cf
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2244
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>