Apply log should be pg_xlog_receive_location() - pg_xlog_replay_location().

Also remove get_last_xlog_replay_timestamp() function and the sql_utils dir,
we can readded later if we need.
We don't need the column last_wal_standby_timestamp in repl_status, so
remove it

repmgr.sql

@@ -12,7 +12,6 @@ CREATE TABLE repl_status(
   last_monitor_timestamp    	 TIMESTAMP WITH TIME ZONE NOT NULL,
   last_wal_primary_location      TEXT NOT NULL,
   last_wal_standby_location      TEXT NOT NULL,
-  last_wal_standby_timestamp	 TIMESTAMP WITH TIME ZONE NOT NULL,
   replication_lag                BIGINT NOT NULL,
   apply_lag                      BIGINT NOT NULL
 );