metadata: merge storage into package

The implementations for the storage of metadata have been merged into a
single metadata package where they can share storage primitives and
techniques. The is a requisite for the addition of namespaces, which
will require a coordinated layout for records to be organized by
namespace.

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

metadata/errors.go

@@ -0,0 +1,17 @@
+package metadata
+
+import "github.com/pkg/errors"
+
+var (
+	ErrExists   = errors.New("metadata: exists")
+	ErrNotFound = errors.New("metadata: not found")
+)
+
+// IsNotFound returns true if the error is due to a missing image.
+func IsNotFound(err error) bool {
+	return errors.Cause(err) == ErrNotFound
+}
+
+func IsExists(err error) bool {
+	return errors.Cause(err) == ErrExists
+}