Evaluate dynamic Gateway-Service association

[canepat]

In order to have separate traffic back to Gateways then the Service needs separate endpoints for them. This can be done in different ways including configuration, a discovery service or by using the Image.sourceIdentity() which can be picked up when a onAvailableImage() callback is triggered: at the moment explicit programming wiring (possibly backed by configuration) is used.
For the request traffic to the same channel and stream id, the Header.sessionId() can be used to match request sources with responses previously matched up with the onAvailableImage() callbacks.

[canepat]

The addService and addGateway APIs in Helios class could be modified to take as parameter just the input stream where the Aeron subscription takes place; the output streams could be added to the Service or Gateway using the Aeron onAvailableImage() callback. The Service and Gateway implementations could be modified to create output ring buffers and their processors accordingly, based on Image callback information (sourceIdentity, streamId, sessionId...).

[canepat]

One simple way to implement two-way-async communication between Gateways and Services is including the response Stream ID in the Gateway messages. Then the Services just send back to individual publishing Gateways and separate via Stream ID.