Make KDL Plugin searchPositionIK actually const, and thread-safe

This fixes the KDL plugin to close https://github.com/ros-planning/moveit_core/issues/47

I've tested the IK on both my robot Maxwell, as well as the PR2 arm+torso (which uses the KDL plugin) and all looks good. I've tested that I can send pick/place requests with many grasps/place-locations without any issue on Maxwell.

The added steps to instantiate the JntArrays and IK/FK solvers in each call to searchPositionIK do not appear slow things down noticeably.