content: remove Provider.Reader

After some analysis, it was found that Content.Reader was generally
redudant to an io.ReaderAt. This change removes `Content.Reader` in
favor of a `Content.ReaderAt`. In general, `ReaderAt` can perform better
over interfaces with indeterminant latency because it avoids remote
state for reads. Where a reader is required, a helper is provided to
convert it into an `io.SectionReader`.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

services/tasks/service.go

@@ -100,11 +100,11 @@ func (s *Service) Create(ctx context.Context, r *api.CreateTaskRequest) (*api.Cr
 		if r.Checkpoint.MediaType != images.MediaTypeContainerd1Checkpoint {
 			return nil, fmt.Errorf("unsupported checkpoint type %q", r.Checkpoint.MediaType)
 		}
-		reader, err := s.store.Reader(ctx, r.Checkpoint.Digest)
+		reader, err := s.store.ReaderAt(ctx, r.Checkpoint.Digest)
 		if err != nil {
 			return nil, err
 		}
-		_, err = archive.Apply(ctx, checkpointPath, reader)
+		_, err = archive.Apply(ctx, checkpointPath, content.NewReader(reader))
 		reader.Close()
 		if err != nil {
 			return nil, err