Handle events where input changes post-ingest or exists into future

The events `DsnComm` and `LoadSegment` contain data that can (and does) change in the iFOT database after having been ingested into Kadi.  The normal process never re-ingests the same event and doesn't allow for events changing or disappearing.  This PR adds two new class-level attributes:

- `lookback_delete`: delete all events after `NOW - lookback_delete` prior to doing daily ingest
- `lookforward`: ingest events out to `NOW + lookforward`