Outbound Transport Partitioning by Key tag

Added optional support for partitioning using a GeoEvent tag as the Producer key
Typical tag used is TRACK_ID, for supporting stateful processing for each track.
Default value is empty string (Kafka round robin partitioning) to preserve backwards compatibility.
Added transport property called "Geoevent Key Tag" to support tag designation.

(This PR was resubmitted as a branch feature instead of an update to master)

Esri / kafka-for-geoevent