Move content store implementation

Move the filesystem content store implementation to a
subpackage of content.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/content.go

@@ -3,20 +3,11 @@ package content
 import (
 	"context"
 	"io"
-	"sync"
 	"time"
 
 	"github.com/opencontainers/go-digest"
 )
 
-var (
-	bufPool = sync.Pool{
-		New: func() interface{} {
-			return make([]byte, 1<<20)
-		},
-	}
-)
-
 type Provider interface {
 	Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error)
 	ReaderAt(ctx context.Context, dgst digest.Digest) (io.ReaderAt, error)