intermediate & final replies, and requests which do not need an immediate reply

[mhaberler]

in the machinekit.io comms stack (zeromq + protobufs), beyond synchronous request-reply we use modified RPC-style patterns which look like so:

• certain requests might not need a reply at all, and hence are marked (a "no RSVP required" in the protobuf message)
• certain requests might cause a series of replies, like intermediate updates, followed by a final response.

I admit it doesnt sound very RPC-ish and maybe a bit whacky, and maybe the better solution is not to use RPCZ for such patterns at all (which we now do), but I would eventually like to funnel all such interactions through the same code path.

I have not yet dug into the code to explore what is needed, if anything.

is this doable? does it make sense to do in the rpcz context? any idea what the effort could be?