repmgrd: optionally disconnect WAL receivers during failover

This is intended to ensure that all nodes have a constant LSN while
making the failover decision.

This feature is experimental and needs to be explicitly enabled with the
configuration file option "standby_disconnect_on_failover".

Note enabling this option will result in a delay in the failover decision
until the WAL receiver is disconnected on all nodes.

repmgr--4.2--4.3.sql

@@ -10,3 +10,8 @@ CREATE FUNCTION get_upstream_last_seen()
   RETURNS INT
   AS 'MODULE_PATHNAME', 'get_upstream_last_seen'
   LANGUAGE C STRICT;
+
+CREATE FUNCTION get_wal_receiver_pid()
+  RETURNS INT
+  AS 'MODULE_PATHNAME', 'get_wal_receiver_pid'
+  LANGUAGE C STRICT;