Include document modification timestamp in DCP events

When using DCP to interface with external systems, it would be useful to be able to propagate the document modification time.

For the Kafka connector, this would allow routing documents to different topics based on the timestamp. For the Elasticsearch connector, this would allow routing to time-based indexes (which has become a hot issue now that ES has stopped supporting document expiration in favor of time-based indexes). For Eventing, it will be useful when event handlers want to do some processing based on when the event actually happened.