message: [c++] Remove last boost::integral_constant uses
  Commit switched from boost::integral_constant to
  std::integral_constant, but didn't make a clean break. With this change,
  everything now uses std::integral_constant.
diff: @@ -782,7 +782,7 @@ get_type_id<int16_t>
  template <> struct
  get_type_id<int32_t>
  - : boost::integral_constant<BondDataType, BT_INT32> {};
  + : std::integral_constant<BondDataType, BT_INT32> {};
  template <> struct
  get_type_id<int64_t>
  @@ -794,7 +794,7 @@ get_type_id<float>
  template <> struct
  get_type_id<double>
  - : boost::integral_constant<BondDataType, BT_DOUBLE> {};
  + : std::integral_constant<BondDataType, BT_DOUBLE> {};
  template <> struct
  get_type_id<void>
  