src.ctf.lttng-live: make lttng_live_stream_iterator::current_msg a bt2::Message:...

src.ctf.lttng-live: make lttng_live_stream_iterator::current_msg a bt2::Message::Shared

Wrapped in an optional, because it is not set at construction, and its
value changes over time.  Adjust a bunch of related functions involved
in creating the message that ends up there.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I2dabd3b3e6d0d17ee2a23774863b160505a0b830
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8458
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>