src.ctf.lttng-live: make lttng_live_trace::trace a bt2::Trace::Shared

src.ctf.lttng-live: make lttng_live_trace::trace a bt2::Trace::Shared

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

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Iadfe9add10e5a6e3112cc0626b2ece868a013b40
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8416
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>