report whether PostgreSQL node is master or standby

 bgw_replstatus is a tiny PostgreSQL background worker to cheaply report the
 replication status of a node. It's intended to be polled by a load balancer
 such as haproxy.
 .
 When installed, a background worker will be started that listens on a TCP
 port. A connection to this port will get a TCP response back saying either
 MASTER or STANDBY depending on the current state of the node.