Revert "Revert "[DBClientUser] Make getUserInfoList(UserInfoList &, const string &) protected, because it doesn't and isn't going to have an access control mechanism.""

This reverts commit ce8f439330801f8dc315b3e8a2315001a7ee2c75.
Revert "[DBClientUser] Define NULL_USER_ID to distinguish it from INVALID_USER_ID."

This reverts commit e86f326a3760c65bbd9aa1e423bf6eb9c0ecb349.
[OperationPrivilege] Add a new method: setFlags().