Pass in a handle and a pair of functions, rather than an object with two
well-known methods. This should make it easier to subscribe to multiple
channels in the future.
but only if the client is modern enough to include "id" in the message,
which lets us avoid sending acks to an 0.7.5 client (which would cause
them to abort, they don't like unrecognized server messages).
The acks let the client learn the server_rx time of messages that
terminate on the server, like "allocate" and "claim".
