ctf: rename bt_msg_iter to ctf_msg_iter

ctf: rename bt_msg_iter to ctf_msg_iter

The bt_msg_iter structure is specific to iterating on CTF messages, so
it would be more appropriate for it to be called ctf_msg_iter.  Rename
it, as well as related types and functions.

Change-Id: I0ce498e492295a3f7390d73d921ef2eca1cc6d00
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2322
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>