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, 8 deletions

diff --git a/sql/rpl_rli.h b/sql/rpl_rli.h
index 84a4aa93cd3..2d92f384ef3 100644
--- a/sql/rpl_rli.h
+++ b/sql/rpl_rli.h
@@ -415,15 +415,8 @@ public:
     Master log position of the event. The position is recorded in the
     relay log info and used to produce information for <code>SHOW
     SLAVE STATUS</code>.
-
-    @param event_creation_time
-    Timestamp for the creation of the event on the master side. The
-    time stamp is recorded in the relay log info and used to compute
-    the <code>Seconds_behind_master</code> field.
   */
-  void stmt_done(my_off_t event_log_pos,
-                 time_t event_creation_time, THD *thd,
-                 rpl_group_info *rgi);
+  void stmt_done(my_off_t event_log_pos, THD *thd, rpl_group_info *rgi);
   int alloc_inuse_relaylog(const char *name);
   void free_inuse_relaylog(inuse_relaylog *ir);
   void reset_inuse_relaylog();