cpp-common/bt2s: control span-lite's contract check based on BT_DEBUG_MODE

cpp-common/bt2s: control span-lite's contract check based on BT_DEBUG_MODE

Enable or disable span-lite's contract checking feature based on
BT_DEBUG_MODE.  When contract checking is enabled and a contract
assertion fails, span-lite calls `std::terminate()`.

Change-Id: Ieba9a08712823daf4b77f135155cd15ec97900f5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12119
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>