message: Fix task-size calculation
diff: #include <string>
  #include "caf/actor.hpp"
  +#include "caf/detail/serialized_size.hpp"
  #include "caf/fwd.hpp"
  #include "caf/intrusive/drr_queue.hpp"
  #include "caf/intrusive/fifo_inbox.hpp"
  @@ -149,8 +150,8 @@ public:
  // nop
  }
  - static constexpr task_size_type task_size(const message&) noexcept {
  - return 1;
  + static task_size_type task_size(const message& msg) noexcept {
  + return detail::serialized_size(msg.msg->content());
  }
  };
  