MQTT transport for capnp RPC

mq.capnp

@0xe425876d9fb9589c

# Disclaimer, I'm no pubsub expert, so there may be inconsitencies and
# other errors in concept or layout with regard to how the queues and
# topics work.

interface Broker {
  publish (message :Message, topic :Topic) -> (result :Result);
  subscribe (subscriber :Subscriber, topic :Topic) -> (result: Result);
}

struct Topic {
  id :Text;
  # ...?
}

struct Message {
  # from: Id; ?
  union {
    text :Text;
    data :Data;
    # ...
  }
}

interface Subscriber {
  handle_message (message :Message, topic :Topic) -> (result: Result);
}

struct Result {
  union {
    ok :Void; # all ok
    failed :Failure;
    # ... other results?
  }

  struct Failure {
    # define failures here..
  }
}