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>

content/content.go

@@ -9,9 +9,14 @@ import (
 	"github.com/opencontainers/go-digest"
 )
 
+type ReaderAt interface {
+	io.ReaderAt
+	io.Closer
+	Size() int64
+}
+
 type Provider interface {
-	Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error)
-	ReaderAt(ctx context.Context, dgst digest.Digest) (io.ReaderAt, error)
+	ReaderAt(ctx context.Context, dgst digest.Digest) (ReaderAt, error)
 }
 
 type Ingester interface {