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/serialize.h

@@ -937,9 +937,9 @@ class CSizeComputer
 protected:
     size_t nSize;
 
+    const int nType;
+    const int nVersion;
 public:
-    int nType;
-    int nVersion;
 
     CSizeComputer(int nTypeIn, int nVersionIn) : nSize(0), nType(nTypeIn), nVersion(nVersionIn) {}