Move content store implementation

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

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

content/helpers.go

@@ -5,12 +5,21 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"sync"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/opencontainers/go-digest"
 	"github.com/pkg/errors"
 )
 
+var (
+	bufPool = sync.Pool{
+		New: func() interface{} {
+			return make([]byte, 1<<20)
+		},
+	}
+)
+
 // ReadBlob retrieves the entire contents of the blob from the provider.
 //
 // Avoid using this for large blobs, such as layers.
@@ -121,8 +130,3 @@ func seekReader(r io.Reader, offset, size int64) (io.Reader, error) {
 
 	return r, errors.Wrapf(errUnseekable, "seek to offset %v failed", offset)
 }
-
-func readFileString(path string) (string, error) {
-	p, err := ioutil.ReadFile(path)
-	return string(p), err
-}