I renamed this to reflect that this flag, when true, will cause the optimal partitioning to be inferred whether that happens to be SP or MP. There is no bias to the inferring, so it doesn't make sense to say that we are trying specifically to infer SP vs. infer MP.

I added m_forceSP which actually distinguishes between the two non-inferred cases -- when m_inferPartitioning == false, and m_forceSP == false, that means that we are actually forcing the plan to be runnable from an MP procedure which usually means it must be an MP plan.

This is another one of those three-valued options like TRUE/FALSE/NULL that are hard to simplify.