message: Make CSerAction's ForRead() constexpr
  The CSerAction's ForRead() method does not depend on any runtime
  data, so guarantee that requests to it can be optimized out by
  making it constexpr.
  Suggested by Cory Fields.
  backports bitcoin@a2929a26f5cf3501804fd12071dd6cf6b464a545
diff: @@ -806,10 +806,10 @@ void Unserialize(Stream& is, std::set<K, Pred, A>& m)
  * Support for ADD_SERIALIZE_METHODS and READWRITE macro
  */
  struct CSerActionSerialize {
  - bool ForRead() const { return false; }
  + constexpr bool ForRead() const { return false; }
  };
  struct CSerActionUnserialize {
  - bool ForRead() const { return true; }
  + constexpr bool ForRead() const { return true; }
  };
  template <typename Stream, typename T>
  