fix regression from moveit#1331 (moveit#1384)

The noted PR swapped boost::thread to std::thread - however
std::thread will cause the program to crash with a note that
TERMINATE CALLED WITHOUT ACTIVE EXCEPTION if the thread object
goes out of scope while not joinable. Detaching the thread
restores the original workflow prior to the PR and allows
users to send more than one trajectory via the motion planning
plugin in RVIZ (otherwise, it crashes on the first trajectory).

moveit_ros/visualization/planning_scene_rviz_plugin/src/planning_scene_display.cpp

@@ -239,6 +239,7 @@ void PlanningSceneDisplay::addBackgroundJob(const std::function<void()>& job, co
 void PlanningSceneDisplay::spawnBackgroundJob(const std::function<void()>& job)
 {
   std::thread t(job);
+  t.detach();
 }
 
 void PlanningSceneDisplay::addMainLoopJob(const std::function<void()>& job)