Make nType and nVersion private and sometimes const

Make the various stream implementations' nType and nVersion private
and const (except in CDataStream where we really need a setter).

src/hash.h

@@ -132,9 +132,9 @@ class CHashWriter
 private:
     CHash256 ctx;
 
+    const int nType;
+    const int nVersion;
 public:
-    int nType;
-    int nVersion;
 
     CHashWriter(int nTypeIn, int nVersionIn) : nType(nTypeIn), nVersion(nVersionIn) {}