message: Add conversion from Timestamp to time_point
diff: @@ -109,7 +109,7 @@ public:
  }
  template <typename C = std::chrono::system_clock, typename D = typename C::duration>
  - std::chrono::time_point<C, D> get_time_point()
  + std::chrono::time_point<C, D> get_time_point() const
  {
  REALM_ASSERT(!m_is_null);
  @@ -119,6 +119,12 @@ public:
  return std::chrono::time_point<C, D>(duration);
  }
  + template <typename C = std::chrono::system_clock, typename D = typename C::duration>
  + explicit operator std::chrono::time_point<C, D>() const
  + {
  + return get_time_point();
  + }
  +
  bool operator==(const Timestamp& rhs) const
  {
  if (is_null() && rhs.is_null())
  