Encapsulate & protect members of CMessageHeader (kinda), CInv, CAddress

CMessageHeader is directly accessed, so the conversion was merely done for non-direct-access callsites.

_Can_ be, yes.  But if this class is sub-classed in the future, that would need to change.

Seems more correct, if unnecessary at present, to use "protected" unless there is a specific reason to use "private".  These members are key to the data structure, and seem likely to be wanted if a subclass ever arises.