Function exchange

One exchange through a transport, with the client's limits enforced whatever the transport does — used by the engine for every hop and by obtainKey:

• timeoutMs (0 = none): the transport gets an AbortSignal that fires at the deadline, and the call rejects then whether the transport stops or not;
• maxResponseBytes (0 = none): checked on the body that came back;
• the response must have an HTTP status, a headers object and a byte body (see HttpResponse); headers come back lower-cased, the body as a Buffer.

A thrown value (a synchronous throw included) and a malformed response reject; the caller turns them into a JobsucheNetworkError (networkError).