Re: Make first dag run optional when catchup is False

[Daniel Standish]

There's some wiggliness here because of Airflow's behavior of actually
*running* the dag at the end of the interval rather than the start.  So if
we have start_date=None, then we default the start date to *now,* then
maybe to be consistent, the first run needs to be not 00:00 tomorrow but
00:00 the next day.  The oddness is amplified when you consider a monthly
dag, where if you deploy now, start date is now, first schedulable run is
next month, therefore first run _more_ than a month away.  To fix this I
think we need to add support in our timetables for running at the start of
the interval instead of the end -- and I think this is something that
timetables were introduced to support anyway.