MDEV-14528 followup.

There was a failure in rpl_delayed_slave after recent MDEV-14528 commit.
The parallel applier should not set its
Relay_log::last_master_timestamp from Format-descriptor log event.
The latter may reflect a deep past so Seconds-behind-master will be
computed through it and displayed all time while the first possibly
"slow" group of events is executed.

The main MDEV-14528 is refined, rpl_delayed_slave now passes also
in the parallel mode.

mysql-test/suite/rpl/disabled.def

@@ -17,4 +17,3 @@ rpl_row_binlog_max_cache_size : MDEV-11092
 rpl_blackhole             : MDEV-11094
 rpl_row_mysqlbinlog       : MDEV-11095
 rpl_row_index_choice      : MDEV-11666
-rpl_delayed_slave         : MDEV-14528

sql/slave.cc

@@ -3962,7 +3962,15 @@ static int exec_relay_log_event(THD* thd, Relay_log_info* rli,
       */
       if (!(ev->is_artificial_event() || ev->is_relay_log_event() || (ev->when == 0)))
       {
-        rli->last_master_timestamp= ev->when + (time_t) ev->exec_time;
+        /*
+          Ignore FD's timestamp as it does not reflect the slave execution
+          state but likely to reflect a deep past. Consequently when the first
+          data modification event execution last long all this time
+          Seconds_Behind_Master is zero.
+        */
+        if (ev->get_type_code() != FORMAT_DESCRIPTION_EVENT)
+          rli->last_master_timestamp= ev->when + (time_t) ev->exec_time;
+
         DBUG_ASSERT(rli->last_master_timestamp >= 0);
       }
     }