Preemting a move_group goal should interrupt planning

[adolfo-rt]

Follow-up of the the preempting planners thread on moveit-users.

The move_group server should react instantly when a cancel request is issued, even if it's currently busy planning. In move_group::MoveGroupMoveAction, planning is executed synchronously with the action server execute callback: executeMoveCallback(...) → executeMoveCallback_Plan*(...) → generatePlan(...)

So implementing this feature would require:

• A threading model in which cancel requests are not blocked by a busy goal callback. A possibility would be to defer planning to a worker thread so the goal callback returns immediately.
• When the MoveGroupMoveAction receives a cancel request and is currently busy planning, preempt the motion planner using PlanningPipeline::terminate()
• Propagate preemption capabilities to the moveit::planning_interface::MoveGroup class. The stop() method looks like a good candidate for handling this. It currently stops execution, but its scope could be extended to stop working, be it planning or executing.