Default users_refresh_interval causes repeated user account loading

MXS-5865: Fix updates of user_refresh_interval clamps users_refresh_time to users_refresh_interval if the former is higher than the latter. Since user_refresh_interval default is 0, users_refresh_time often ends up as 0 as well, ignoring the setting value.

To fix this, user_refresh_interval value 0 should be ignored. Also, in general the situation where users_refresh_interval != 0 and users_refresh_interval < users_refresh_time could be considered a config error.