Bugfix: make joint bound checks threadsafe

[axelschroth]

Use a (thread local) copy of robot state for checking joint bounds instead of a pointer to the same robot state instance. Since robot state gets modified during bound check, good solutions may have been discarded when using IK in multiple threads concurrently.

[simonschmeisser]

Thanks for your endurance @axelschroth and thanks for finally finding the root of our strange rare failures when planning cartesian paths.