allow the server to asynchronously pass errors back, while still
allowing streamed and pipelined data.
+Unfortunately, the multiplex protocol is not used at every stage. We
+start up in plain socket mode and then change over by calling
+io_start_buffering. Of course both the client and the server have to
+do this at the same point.
+
The server then talks to the client as normal across the socket,
passing checksums, file lists and so on. For documentation of that,
stay tuned (or write it yourself!).
+
+