Medium: pgsql: set only one node into sync mode when using 3 nodes or higher

Only one node can be sync mode to avoid data loss when using 3 nodes or higher.
See http://www.gossamer-threads.com/lists/linuxha/pacemaker/85077?do=post_view_threaded#85077
Low: pgsql: remove unnecessary code which is used by multiple synced nodes
Low: pgsql: re-control slave status when sync mode is changed

execute control_slave_status() once again to improve performance
of catching up slave status.