Tweak setDefaults API

[stephenh]

We should separate:

• "sync defaults" that are set immediately on em.create, from
• "async defaults" that only run on em.flush

We should allow a sync default that can read other FKs, i.e.:

/** Ensure that the iti has an originalIti set on creation */
config.beforeCreate((iti) => {
  if (!iti.original.isSet) {
    iti.original.set(iti);
  }
});

To:

config.setDefault("original", iti => iti.original.get ?? iti);