Provide service response to original service client using client GID

[christophebedard]

Register clients in the ServiceHandler with client GID instead of service name. Then when figuring out which ServiceClient to give a service response to, get the original client GID from the headers.

This means we have to keep the client GID between the taking of the request and the sending of the resposne. This is why rmw combines sequence ID and "writer GUID" into a "request ID" struct.