iostream: use new-style consumer to implement copy()

in 62dd712, we added two concept for both the old-style
consumer which returns `std::optional<temporary_buffer>`
consumer, and the new one which returns `consumption_result`.
but somehow, we are still using the old-style in the implementation
of `copy()`.

in this change, we switch to the new-style consumer. this helps
developers to understand the usage of this consumer, and also
enables us to drop the obsolete interface in future.

Signed-off-by: Kefu Chai <[email protected]>

include/seastar/core/iostream-impl.hh

@@ -527,16 +527,16 @@ template <typename CharType>
 struct stream_copy_consumer {
 private:
  output_stream<CharType>& _os;
- using unconsumed_remainder = std::optional<temporary_buffer<CharType>>;
+ using consumption_result_type = consumption_result<CharType>;
 public:
  stream_copy_consumer(output_stream<CharType>& os) : _os(os) {
  }
- future<unconsumed_remainder> operator()(temporary_buffer<CharType> data) {
+ future<consumption_result_type> operator()(temporary_buffer<CharType> data) {
  if (data.empty()) {
- return make_ready_future<unconsumed_remainder>(std::move(data));
+ return make_ready_future<consumption_result_type>(stop_consuming(std::move(data)));
  }
- return _os.write(data.get(), data.size()).then([] () {
- return make_ready_future<unconsumed_remainder>();
+ return _os.write(data.get(), data.size()).then([] {
+ return make_ready_future<consumption_result_type>(continue_consuming());
  });
  }
 };