code: function confChanged(current, proposed) {
      if (stringify(current.conf) !== stringify(proposed.conf)) {
          if (current.version >= proposed.version) {
              const e = new Error('Schema change, but no version increment.');
              e.current = current;
              e.proposed = proposed;
              throw e;
          }
          return true;
      } else {
          return false;
      }
  }
docs: Check if a schema part differs, and if it does, if the version was
  incremented.
  @param {Object} current { conf: {object}, version: {number} }
  @param {Object} proposed { conf: {object}, version: {number} }
  @return {boolean} Whether the attribute has changed.
  @throws {Error} If the schema fragment changed, but the version was not
  incremented.