planning_scene_monitor: fix state update bug

The rate of state updates is limited to dt_state_update_ per second.
When an update arrived it was not processed if another was recently
processed.  This meant that if a quick sequence of state updates
arrived and then no updates arrive for a while that the last update(s)
were not seen until another arrives (which may be much later or
never). This fixes the bug by periodically checking for pending
updates and running them if they have been pending longer than
dt_state_update_.

Fixes issue https://github.com/ros-planning/moveit_ros/issues/405 (replaces pull request https://github.com/ros-planning/moveit_ros/pull/407)