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>

remotes/docker/schema1/converter.go

@@ -210,13 +210,13 @@ func (c *Converter) fetchBlob(ctx context.Context, desc ocispec.Descriptor) erro
 		// TODO: Check if blob -> diff id mapping already exists
 		// TODO: Check if blob empty label exists
 
-		r, err := c.contentStore.Reader(ctx, desc.Digest)
+		ra, err := c.contentStore.ReaderAt(ctx, desc.Digest)
 		if err != nil {
 			return err
 		}
-		defer r.Close()
+		defer ra.Close()
 
-		gr, err := gzip.NewReader(r)
+		gr, err := gzip.NewReader(content.NewReader(ra))
 		if err != nil {
 			return err
 		}