Fix planning scene display initial state

When rviz starts after move_group, the planning scene diff messages sent by move_group do not give PlanningSceneDisplay the full state of the planning scene.  For example if the robot has not moved for a while then move_group may not be sending updates of the robot state and the state of the PlanningScene robot will be different from the actual state of the real robot (it will appear in the default position rather than the current position).

This change fixes the PlanningSceneDisplay so that when it starts up (robot model is loaded or planning scene monitor topic changes) it will query move_group for the current full PlanningScene state and use that to initialize the PlanningScene maintained by the PlanningSceneDisplay.

Fixes issue https://github.com/ros-planning/moveit_ros/issues/405
Replaces https://github.com/ros-planning/moveit_ros/pull/407 and adds to https://github.com/ros-planning/moveit_ros/pull/408