Deliver the `ack` promise with the result of attempting to write to kafka. The
default command-handler waits for this before on to the next command so the
test response may indicate the success/failure of each write command.

Creates an asynchronous Kafka Consumer of all topics defined in the
supplied `topic-metadata`

Puts all messages on the channel in the returned response. It is the
responsibility of the caller to arrange for the read the channel to
be read by some other process.

Must be closed with `close-consumer` when no longer required

Returns a function that takes a consumer and puts any messages retrieved
by polling it onto the supplied `messages` channel

Creates an asynchronous kafka producer to be used by a test-machine for for
injecting test messages