Add config for objectId size (#3950)

* Add objectId config property, default to 10

* Update Config constructor

* Add test for backwards compatibility when changing objectId size

src/defaults.js

@@ -32,5 +32,6 @@ export default {
   expireInactiveSessions: true,
   revokeSessionOnPasswordReset: true,
   schemaCacheTTL: 5000, // in ms
-  userSensitiveFields: ['email']
+  userSensitiveFields: ['email'],
+  objectIdSize: 10
 }