message: simplify duration template
diff: #pragma once
  +#include <chrono>
  #include <set>
  #include <string>
  #include <thread>
  @@ -87,10 +88,8 @@ public:
  /// Resolves a path to a remote actor.
  void resolve(const uri& locator, const actor& listener);
  - template <class Handle = actor, class Rep, class Period>
  - expected<Handle>
  - remote_actor(const uri& locator,
  - std::chrono::duration<Rep, Period> timeout_duration
  + template <class Handle = actor, class Duration = std::chrono::seconds>
  + expected<Handle> remote_actor(const uri& locator, Duration timeout_duration
  = std::chrono::seconds(5)) {
  scoped_actor self{sys_};
  resolve(locator, self);
  