Future PR16849 (Dxxxx) introduces locking `m_cs_chainstate` to their version of `InvalidateBlock`. Our codepath for that is shared by `ParkBlock`, `InvalidateBlock` and `FinalizeBlock`, so having all three be members of `CChainState` (`FinalizeBlock` already is) makes sense.
Also made `UnwindBlock` private since it's not supposed to be called by just anyone, rather by stuff that knows what they're doing -, and moved `FinalizeBlock` implementation to be closer to the other two since that makes more sense.