Rename pc_msg_iter fields to self_msg_iter

Rename pc_msg_iter fields to self_msg_iter

The pc prefix means "private connection", which is not a concept
present in the API anymore.  Rename to "self_msg_iter", which is a more
contemporary term.

Change-Id: I76ab362cab05d1dd480549fc285060854375e1e7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2321
CI-Build: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>