ctf: use self component / message iterator C++ wrappers throughout

ctf: use self component / message iterator C++ wrappers throughout

Use `bt2::SelfComponent`, `bt2::SelfSourceComponent`,
`bt2::SelfSinkComponent` and `bt2::SelfMessageIterator` for function
parameters and fields throughout `src/plugins/ctf` instead of raw
pointers.

Don't try to adapt the code too much to use the wrappers yet.

Change-Id: I2a3ae5849ef44fd639486185e5718919a152be93
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12416
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>