Change the OSC structure to make it fully compliant with the systems framework.

[Brian-Acosta]

Some notes:

• Separated the state of each tracking data from the structural information to allow for storage as a CacheEntry
• Removed the spring/no spring copies from OscTrackingData, since they were already removed from the OSC
• The OSC class still has a mutable InverseDynamicsQp member. The state of the QP is fully defined by UpdateDynamics, which is called from the same function where the program is solved, so the behavior is correct.
• Changed double support blending constraint to a cost, consistent with active locomotion-specific branches

@yangwill, can you test the correctness of the impact invariant implementation?

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