function makeDb(config, instanceAlias = 'default') {
  // makeDb should only be called once for a given alias
  if (instances[instanceAlias]) {
    throw new Error(`db instance ${instanceAlias} already made`);
  }
  const dbPromise = initDb(config);
  instances[instanceAlias] = dbPromise;
  return true;
}
Initializes a db instance for a given config.
Ensures that this only happens once for a given alias.
If this were called multiple times weird things could happen
@param {object} config
@param {string} instanceAlias