message: streams: Add data() method to CDataStream
  Analogous to c++11 vector data().
diff: @@ -178,6 +178,8 @@ public:
  void clear() { vch.clear(); nReadPos = 0; }
  iterator insert(iterator it, const char& x=char()) { return vch.insert(it, x); }
  void insert(iterator it, size_type n, const char& x) { vch.insert(it, n, x); }
  + value_type* data() { return vch.data() + nReadPos; }
  + const value_type* data() const { return vch.data() + nReadPos; }
  void insert(iterator it, std::vector<char>::const_iterator first, std::vector<char>::const_iterator last)
  {
  