ctf: const-ify a few bt_message parameters

I didn't understand at first why we needed a temporary local variable in
ctf_fs_iterator_next_one, why we couldn't just pass out_msg directly to
ctf_fs_ds_file_next.  If we constify the parameter of
ctf_fs_ds_file_next, we can get rid of the variable.  Doing so has a few
ramifications, it requires to constify the parameter in a few other
functions.  But in the end I think it's all for the greater good.

Change-Id: Ie68249d38dae7a26ab8ce9341b436257644b3b10
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>