"standby switchover": avoid potential race condition with WAL location check

Immediately after the demotion candidate (primary) has shut down, we can't
be absolutely sure that the walreceiver has flushed all WAL to disk, so
checking pg_last_wal_receive_lsn() at that point might not reflect
the actual last available WAL location.

To handle this, we'll loop for a while (timeout controlled by configuration
parameter "wal_receive_check_timeout") before finally deciding whether
the standby is still behind the shut-down primary.

Addresses issue raised in GitHub #518.

configfile.h

@@ -106,6 +106,7 @@ typedef struct
 	/* standby switchover settings */
 	int			shutdown_check_timeout;
 	int			standby_reconnect_timeout;
+	int			wal_receive_check_timeout;
 
 	/* node rejoin settings */
 	int			node_rejoin_timeout;
@@ -189,6 +190,7 @@ typedef struct
 		/* standby switchover settings */ \
 		DEFAULT_SHUTDOWN_CHECK_TIMEOUT, \
 		DEFAULT_STANDBY_RECONNECT_TIMEOUT, \
+		DEFAULT_WAL_RECEIVE_CHECK_TIMEOUT, \
 		/* node rejoin settings */ \
 		DEFAULT_NODE_REJOIN_TIMEOUT, \
 		/* node check settings */ \