Timestamp is recommended over received_at when chronology matters

[jimmyocto]

The Segment docs recommend using timestamp for historical analysis when chronology matters. In the case of this demo segment code we are ordering events within sessions. We should consider using timestamp instead of received_at.

https://segment.com/docs/spec/common/#timestamps

The tradeoff seems to be speed. Looks analyzing sessions seem to become much slower when we make this change.