Replace agent sections

Make the agent's sections exactly the list sent, in that order. Existing sections are matched by name: a matched section keeps its ID and is changed only if its content or place differs, new names are added, and sections missing from the list are deleted. A request that changes nothing writes nothing, so it is safe to send on every deploy. Returns the sections in their new order. Returns 400 when the list repeats a name, 404 when the agent is not found, and 409 when the agent already has several sections with one name.