Problem:
========
A slave’s relay log format description event is used when
calculating Seconds_Behind_Master (SBM). This forces the SBM
value to spike when processing these events, as their creation
date is set to the timestamp that the IO thread begins.

Solution:
========
When the slave generates a format description event, mark the
event as a relay log event so it does not update the
rli->last_master_timestamp variable.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>