Parse and store the game's duration.

Parse and store the game's duration.

The duration has been sent for a while as the 'D' line, but
it hasn't really been needed until now; it was just a nice thing
to have. Now Elo calculations are messing up because the warmup
time is being summed into player alivetimes. This duration field
can assist in creating a workaround. Having it, Elo calculations
can know the true maximum game time, so anything higher can be
brought down to the "floor" of the real duration of the match.