Step 3.4, first half. Every one of these took an *amqp.Channel, so the
transport reached every caller and swapping it meant touching all of them.
The seam turned out to be small — the controller sends exactly two kinds of
message that expect no answer — which is the same measurement that said
this bus could be replaced at all.
Bus is stated in the mesh's words, not a transport's: PublishEvent and
PublishDeclaration. Two implementations, both shipping, because steps 1 to
4 leave every node on AMQP and the NATS one is selected at the rollout.
Both ship is also what makes them comparable: one conformance fixture holds
both to the same envelope, and the NATS one is checked against a real
server reading back from the stream rather than from the code that wrote it.
Still on *amqp.Channel: RequestBuild and Ask, which carry reply-queue
machinery, and the whole consume side — the control loop, enrolment, serve.
