SU - remove GRANT'able permissions and refactor to ROLE attributes

* Add SETROLE which replaces ADMIN - Allow user to SET ROLE to any
    non-superuser role.
* Add READONLY - Allow user to ONLY view data, disallow INSERT.
* Add BACKUP - Allow user to perform backup operations.
* Add GRANT - Allow user to perform GRANT and CREATE POLICY.
* Add LOGROTATE - Allow user to rotate log files.
* Add MONITOR - Allow user to view details from pg_stat_* functions.
* Add PROCSIGNAL - Allow user to signal other backend processes -
    specifically allows terminate/cancel backends.