MTS & ROTATING RELAY-LOG

    Problem: The value of 'Seconds_Behind_Master' (SBM) reports
    distorted number when rotating a relay-log.

    Analysis: The issue is happening only on MTS enabled Slaves.
    The distorted number is coming while the Slave is executing
    fake rotate events. Fake rotate event (=which does not exist
    physically in any binlog) is a rotate event, which contains
    the name of the binlog Master is going to send to the slave
    (because the slave may not know the binlog name if it just
    asked for MASTER_LOG_FILE='', MASTER_LOG_POS=4). These events
    contains zero in it's even time stamp value which will be
    replaced by Slave's during the execution with I/O thread's
    time. If SBM is calculated while these fake rotate events
    are executing, the SBM value end up in getting big number
    which confuses the users.

    Fix: For Single thread slaves, Slave server is already
    ignoring to set the last master_time stamp (which is used
    to calculate SBM) if the event is fake/artificial event by
    using is_artificial_event() function. Now with this patch,
    Slave will ignore these fake/artificial rotate events even
    in MTS execution path as well.