Add children bucket to back reference snapshots

Adds back links from parent to children in order to prevent
deletion of a referenced snapshot in a namespace.
Avoid removing snapshot during metadata delete to
prevent shared namespaces from being mistakenly deleted.

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

snapshot/storage/bolt.go

@@ -305,7 +305,7 @@ func Remove(ctx context.Context, key string) (string, snapshot.Kind, error) {
 		if pbkt != nil {
 			k, _ := pbkt.Cursor().Seek(parentPrefixKey(id))
 			if getParentPrefix(k) == id {
-				return errors.Errorf("cannot remove snapshot with child")
+				return errors.Wrap(errdefs.ErrFailedPrecondition, "cannot remove snapshot with child")
 			}
 
 			if si.Parent != "" {