We add a column for identify mode before for identify az, which not
compatible with distributed gaussdb. Therefore, we remove the added
column in identify mode, and add another command identify az to make
sure a cascade standby and its upstream both in same available zone.
We support cascade standby now, a cascade standby will get WAL from
a standby asynchronously. To support flexible deployment, we add a
new GUC available_zone to identify the available zone of a cascade
standby, which will try to connect a standby in same available zone.
Users can use utility gs_ctl for failover and switchover. A cascade
standby will be promoted to be a standby when failover or switchover,
while a standby will be demoted to be a cascade standby after switchover.
