Report claims that Seconds_behind_master behaves unexpectedly. 

Code analysis shows that there is an evident flaw in that treating of FormatDescription event is wrong
so  that after FLUSH LOGS on slave the Seconds_behind_master's calculation slips and incorrect
value can be reported to SHOW SLAVE STATUS. 
Even worse is that the gap between the correct and incorrect deltas grows with time.

Fixed with prohibiting changes to rpl->last_master_timestamp by artifical events (any kind of).
suggestion as comments is added how to fight with lack of info on the slave side by means of
new heartbeat feature coming.

The test can not be done ealily fully determistic.