Make chng report max lag from drop date, not datetime.now()

[krivard]

The chng indicator (and perhaps others too) does this:

covidcast-indicators/changehc/delphi_changehc/run.py

Line 208 in ac10f61

max_lag_in_days = min_max_date and (datetime.now() - min_max_date).days

This produces alarming output ("max lag: 154 days") when running patches, since the patch lag should have max lag <50 or so.

We should change how max lag is computed to be based off the drop_date param instead of datetime.now()

[aysim319]

the source is no longer active

[melange396]

the chng signal is supposed to come back at some point, we should keep this around until then (or until we determine give up on waiting)