Replace the Kafka Gateway with a new Kafka-to-REST adapter service

[wajda]

It will make individual services more narrow focused and specialized. Kafka-to-REST bridge will simply pump Spline messages from a Kafka to the Producer REST API, that will become the main gate and will decouple Spline persistence layer from the transport layer.

[wajda]

After some discussions it is now not clear if it's actually a desired change. It would split the code nicely, but it could potentially add more latency due to longer message route. So I'll put it aside for now.