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>

images/image.go

@@ -3,7 +3,6 @@ package images
 import (
 	"context"
 	"encoding/json"
-	"io/ioutil"
 	"time"
 
 	"github.com/containerd/containerd/content"
@@ -73,13 +72,7 @@ func Config(ctx context.Context, provider content.Provider, image ocispec.Descri
 	return configDesc, Walk(ctx, HandlerFunc(func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {
 		switch image.MediaType {
 		case MediaTypeDockerSchema2Manifest, ocispec.MediaTypeImageManifest:
-			rc, err := provider.Reader(ctx, image.Digest)
-			if err != nil {
-				return nil, err
-			}
-			defer rc.Close()
-
-			p, err := ioutil.ReadAll(rc)
+			p, err := content.ReadBlob(ctx, provider, image.Digest)
 			if err != nil {
 				return nil, err
 			}