Improved the SimpleActionServer interface and refactored DWB's main control loop (#849)

Extended the SimpleActionServer interface and improved a few other things:
* Added additional checks to make sure we only keep the latest preempt request in the queue.
* Action server users no longer interact directly with the goal handles. 
* Provided additional functions for aborting, canceling, etc.
* Updated bt_navigator, motion_primitives, dwb_controller, navfn_planner to use the new interface.

Refactored the DWB controller main loop.
* Added checks for handling pending preemptions after the controller has succeeded or failed.