src.ctf.lttng-live: introduce live_viewer_connection::UP and use it

src.ctf.lttng-live: introduce live_viewer_connection::UP and use it

Change lttng_live_msg_iter::viewer_connection to be a
live_viewer_connection::UP and convert live_viewer_connection_create's
output parameter to that type.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I0f91d4636c67ad9f820783932fa3c45464dfb3bb
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8472
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12387
Tested-by: jenkins <jenkins@lttng.org>