Add utils::static_unique_ptr_cast()

Also, replace all static_cast<T*>(base.get()) constructs by
utils::static_unique_ptr_cast<T>(std::move(base)).
For example:
    auto request =
        static_cast<api::iotask_create_request*>(base_request.get());
becomes:
    auto request =
        utils::static_unique_ptr_cast<api::iotask_create_request>(
            std::move(base_request));