postgresql: Fix one-by-one error in db_to_date on negative dates

Using int() on date.timestamp() rounded it up (toward zero),
but the semantics of model.Timestamp is that the actual time is
ts.seconds + ts.microseconds/1000000, so all negative dates were
shifted one second up.

In particular, this causes dates from
1969-12-31T23:59:59.000001 to 1969-12-31T23:59:59.999999
(inclusive) to smash into dates from
1970-01-01T00:00:00.000001 to 1970-01-01T00:00:00.999999,
which is how I discovered the issue.