Low: pgsql: improve error handling of crm_xxxx commands or reload_conf()

- retry executing crm_xxxx commands if they fail,
  because the number of IPC connections reaches maximum,
  crm_xxxx commands fail.
- if reload_conf() fails, retry it, because the failure is critical
Low: pgsql: report failed query and return code
Low: pgsql: simplify picking out data status
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: control slave status once again when sync mode is changed

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