repmgrd: during failover, check if a node was already promoted

Previously, repmgrd assumed that during a failover, there would not
already be another primary node. However it's possible a node was
promoted manually. While this is not a desirable situation, it's
conceivable this could happen in the wild, so we should check for
it and react accordingly.

Also sanity-check that the follow target can actually be followed.

Addresses issue raised in GitHub #420.

dbutils.h

@@ -302,6 +302,7 @@ typedef struct BdrNodeInfoList
 typedef struct
 {
 	char		current_timestamp[MAXLEN];
+	bool		in_recovery;
 	XLogRecPtr	last_wal_receive_lsn;
 	XLogRecPtr	last_wal_replay_lsn;
 	char		last_xact_replay_timestamp[MAXLEN];