standby follow: don't restart server (PostgreSQL 13 and later)

As of PostgreSQL 13, changes to the fundamental replication
configuration can be applied with a simple SIGHUP, no restart
required.

In case the old behaviour is desired, i.e. a full restart to apply
the configuration changes, the new configuration parameter
"standby_follow_restart" can be set. This parameter has no effect
in PostgreSQL 12 and earlier.

configfile.h

@@ -164,6 +164,7 @@ typedef struct
 	/* standby follow settings */
 	int			primary_follow_timeout;
 	int			standby_follow_timeout;
+	bool		standby_follow_restart;
 
 	/* standby switchover settings */
 	int			shutdown_check_timeout;