MDEV-5114 seconds_behind_master flips to 0 & spikes back, when running show slaves status

1. After a period of wait (where last_master_timestamp=0)
   do NOT restore the last_master_timestamp to the timestamp
   of the last executed event (which would mean we've just
   executed it, and we're that much behind the master).

2. Update last_master_timestamp before executing the event,
   not after.

Take the approach from the this commit (but with a different test
case that actually makes sense):

commit 0c75ab453fb8c5439576af8fe5add7a1b89f1569
Author: Luis Soares <luis.soares@sun.com>
Date:   Thu Apr 15 17:39:31 2010 +0100

    BUG#52166: Seconds_Behind_Master spikes after long idle period

1 files changed, 1 insertions, 21 deletions

diff --git a/sql/rpl_rli.cc b/sql/rpl_rli.cc
index a324c3c30da..9bd0ca55b01 100644
--- a/sql/rpl_rli.cc
+++ b/sql/rpl_rli.cc
@@ -1290,13 +1290,9 @@ bool Relay_log_info::is_until_satisfied(THD *thd, Log_event *ev)
 }
 
 
-void Relay_log_info::stmt_done(my_off_t event_master_log_pos,
-                               time_t event_creation_time, THD *thd,
+void Relay_log_info::stmt_done(my_off_t event_master_log_pos, THD *thd,
                                rpl_group_info *rgi)
 {
-#ifndef DBUG_OFF
-  extern uint debug_not_change_ts_if_art_event;
-#endif
   DBUG_ENTER("Relay_log_info::stmt_done");
 
   DBUG_ASSERT(rgi->rli == this);
@@ -1350,22 +1346,6 @@ void Relay_log_info::stmt_done(my_off_t event_master_log_pos,
     if (mi->using_gtid == Master_info::USE_GTID_NO)
       flush_relay_log_info(this);
     DBUG_EXECUTE_IF("inject_crash_after_flush_rli", DBUG_SUICIDE(););
-    /*
-      Note that Rotate_log_event::do_apply_event() does not call this
-      function, so there is no chance that a fake rotate event resets
-      last_master_timestamp.  Note that we update without mutex
-      (probably ok - except in some very rare cases, only consequence
-      is that value may take some time to display in
-      Seconds_Behind_Master - not critical).
-
-      In parallel replication, we take care to not set last_master_timestamp
-      backwards, in case of out-of-order calls here.
-    */
-    if (!(event_creation_time == 0 &&
-          IF_DBUG(debug_not_change_ts_if_art_event > 0, 1)) &&
-        !(rgi->is_parallel_exec && event_creation_time <= last_master_timestamp)
-        )
-        last_master_timestamp= event_creation_time;
   }
   DBUG_VOID_RETURN;
 }