pgsql: fix failback operation refs #33389

repmgr v4.2 (on debian 10) has a bug, it doesn't detect role change after back to normal state: https://github.com/2ndQuadrant/repmgr/issues/524#issuecomment-459230305. The impact is after 1st successful switchover and failback repmgr will do no more switchover until restarted. This commit adds a restart operation to repmgrd (automatically executed on standby-to-primary tag).