async function getDb(instanceAlias = 'default') {
  const instancePromise = instances[instanceAlias];
  if (!instancePromise) {
    throw new Error('db instance must be created first');
  }
  // already be a promise -- this just makes it explicit
  const { models, sequelizeDb } = await instancePromise;
  return { models, sequelizeDb };
}
Returns promise of db instance
@param {string} [instanceAlias]