[exim] queue should be FIFO not LIFO

Gentlemen, examining /var/log/exim4/mainlog shows the queue is last in
first out, at least with my single smarthost. Any simple variable to
flip to make it first in first out?

Imagine if the postal service was LIFO. Granny's card might never get
tended to before the close of business today, as it sits on the bottom
of the pile.