message: Remove stale reference to legacy duration type
diff: #include "caf/mixin/requester.hpp"
  #include "caf/mixin/sender.hpp"
  #include "caf/scheduled_actor.hpp"
  +#include "caf/timespan.hpp"
  #include "caf/typed_actor_view_base.hpp"
  namespace caf {
  @@ -106,7 +107,7 @@ public:
  return self_->new_request_id(mp);
  }
  - void request_response_timeout(const duration& d, message_id mid) {
  + void request_response_timeout(timespan d, message_id mid) {
  return self_->request_response_timeout(d, mid);
  }
  