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>

metadata/buckets.go

@@ -45,6 +45,7 @@ var (
 	bucketKeyRuntime     = []byte("runtime")
 	bucketKeyName        = []byte("name")
 	bucketKeyParent      = []byte("parent")
+	bucketKeyChildren    = []byte("children")
 	bucketKeyOptions     = []byte("options")
 	bucketKeySpec        = []byte("spec")
 	bucketKeySnapshotKey = []byte("snapshotKey")