snapshot: define the snapshot driver interface

We now define the `snapshot.Driver` interface based on earlier work.
Many details of the model are worked out, such as snapshot lifecycle and
parentage of commits against "Active" snapshots.

The impetus of this change is to provide a snapshot POC that does a
complete push/pull workflow. The beginnings of a test suite for snapshot
drivers is included that we can use to verify the assumptions of
drivers. The intent is to port the existing tests over to this test
suite and start scaling contributions and test to the snapshot driver
subsystem.

There are still some details that need to be worked out, such as listing
and metadata access. We can do this activity as we further integrate
with tooling.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

content/content_test.go

@@ -16,12 +16,14 @@ import (
 	"testing"
 	"time"
 
+	"github.com/docker/containerd/testutil"
 	"github.com/opencontainers/go-digest"
 )
 
 func TestContentWriter(t *testing.T) {
 	tmpdir, cs, cleanup := contentStoreEnv(t)
 	defer cleanup()
+	defer testutil.DumpDir(t, tmpdir)
 
 	if _, err := os.Stat(filepath.Join(tmpdir, "ingest")); os.IsNotExist(err) {
 		t.Fatal("ingest dir should be created", err)
@@ -114,7 +116,6 @@ func TestContentWriter(t *testing.T) {
 		t.Fatal("mismatched data written to disk")
 	}
 
-	dumpDir(tmpdir)
 }
 
 func TestWalkBlobs(t *testing.T) {
@@ -279,14 +280,3 @@ func checkWrite(t checker, cs *Store, dgst digest.Digest, p []byte) digest.Diges
 
 	return dgst
 }
-
-func dumpDir(root string) error {
-	return filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
-
-		fmt.Println(fi.Mode(), path)
-		return nil
-	})
-}