Back off from a link that dies as soon as it comes up

A bind that returns is not proof the link works: an unreadable stream is
only found afterwards, so resetting the delay there handed a link that
died on arrival a fresh minDelay every cycle — one connect and bind per
second forever, which is how an account gets blocked for bind flooding.
The loop resets only once a link has outlasted maxDelay.

Also covers the unreadable stream that has no loop to retry it.

src/pdu-transport.ts

@@ -14,7 +14,7 @@ export type PduTransportOptions = {
 	/** A complete PDU, before it is parsed. */
 	onFramed: (pdu: Buffer) => void;
 	onPdu: (pduObj: PduObject) => void;
-	/** Nothing further can be read off this stream; the socket has to go.  */
+	/** Nothing further can be read off this stream, whatever the socket does next. */
 	onUnreadable: (err: Error) => void;
 };