src.ctf.lttng-live: make lttng_live_trace::trace_class a bt2::TraceClass::Shared

src.ctf.lttng-live: make lttng_live_trace::trace_class a bt2::TraceClass::Shared

Use the C++ wrapper to hold the reference to the trace class.  Wrap it
in an optional, since it's not assigned at creation.

Note that the existing comment said it is a weak reference, but the code
shows otherwise.

Change-Id: Ic486a65ab40953c5d6a3ddb7d057d2caa85cf1c0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8415
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>