In T240460#6614767 we decided the following:
- `dt` is always a client AKA event timestamp.
- `meta.dt` is always a server side receive timestamp.
- Which timestamp field is used for Kafka index and Hive partitioning is configurable.
To accomplish this:
 All schemas should be updated to have both a `meta.dt` and a `dt` field, with neither field required
 eventgate-*-external instances should use `meta.dt` as the Kafka timestamp
 eventgate-*-external Camus jobs should be configured to use `meta.dt` for hourly HDFS partitioning
 eventgate-* internal instances should use `dt` as the Kafka timestamp, falling back to `meta.dt`
 eventgate-* internal Camus jobs should be configured to use `dt` for hourly HDFS partitioning, falling back to `meta.dt`
 EventBus should be modified to set `dt`, but not `meta.dt` (allowing EventGate to fill it in)
 Other event producers (change-prop?) that don't use EventGate should set `dt` but not `meta.dt`
(In the future when we get rid of Camus, I'd expect to be able to use the Kafka message timestamp for hourly partitioning, and not worry about having specific timestamp field configuration for HDFS ingestion hourly partitioning.)