Add an option for STANDBY FOLLOW to wait for a master to appear.

This is important for autofailover to do the right thing when
standbys detected master death at different times.

While this is a new option, seems important for the autofailover
to work properly so i will consider the lack of it a bug and
will backpatch to 2.0 where autofailover was introduced.

For gripe from Alex Railean, about a standby not finding the new
master because the new master hasn't finish promoting.

repmgr.h

@@ -59,6 +59,7 @@ typedef struct
 	char wal_keep_segments[MAXLEN];
 	bool verbose;
 	bool force;
+	bool wait_for_master;
 	bool ignore_rsync_warn;
 
 	char masterport[MAXLEN];