Fix follow master command to skip already-dead node.
Fix follow master command to skip already-dead node.

For example, if primary node crashed and one of standby nodes is
promoted, follow master command should not automatically recover the
old primary node because it might be physically broken. For this
purpose, pg_ctl status is called to check the DB node is dead or
alive.

treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres