src.ctf.fs: make ctf_msg_iter_create return a unique_ptr

src.ctf.fs: make ctf_msg_iter_create return a unique_ptr

Introduce ctf_msg_iter_up, a unique_ptr type with a deleter that calls
ctf_msg_iter_destroy.  Make ctf_msg_iter_create return that.

Change-Id: Ibba15a84c58804a88f18c6ed47b9197fe46541b3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8275
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12313
Tested-by: jenkins <jenkins@lttng.org>