message: Define timestamp in terms of a timespan
diff: namespace caf {
  +/// A portable timespan type with nanosecond resolution.
  +using timespan = std::chrono::duration<int64_t, std::nano>;
  +
  /// A portable timestamp with nanosecond resolution anchored at the UNIX epoch.
  -using timestamp = std::chrono::time_point<
  - std::chrono::system_clock,
  - std::chrono::duration<int64_t, std::nano>
  ->;
  +using timestamp = std::chrono::time_point<std::chrono::system_clock, timespan>;
  /// Convenience function for returning a `timestamp` representing
  /// the current system time.
  