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Use the newer version of the bullet 6dof spring constraint for rigidbody.

The new constraint is slower and not backward compatible, but should
be better, especially in the damping side. The new constraint also
has a different valid range of the damping coefficient, and a limit
implementation that bounces instead of making the object stationary.

Reviewers: sergof

Differential Revision: https://developer.blender.org/D3125