the relay log before flushing master.info.
Doing 'before' leads to duplicate event, doing after leads to missing event.
Both can be as destructive, but 'duplicate' enables us to later add detection
code to catch it. Whereas 'missing' can't be caught (it can't, because
the I/O thread can produce legal position jumps, for example if it has
ignored an event coming from this slave (rememember that starting from 4.1.1,
the I/O thread filters the server id).