Client/service identifier and service request header format

[eboasson]

Client requests and service responses are serialized by prefixing the serialization of the actual content of the request/response by a request id consisting of the local instance handle of the partiicpant and a 64-bit sequence number.

The way the instance handles are generated means that it is highly likely to be unique, but there is no guarantee, and perhaps it would be better to use the GUID.

Also it would be nice if it matched the format used by other RMW layers to have interoperability, but I don't know what the others do, nor whether there's agreement what it should be.