Only send one immediate ACK

Fixes #3357

draft-ietf-quic-transport.md

@@ -3136,12 +3136,10 @@ This recommendation is in keeping with standard practice for TCP {{?RFC5681}}.
 
 In order to assist loss detection at the sender, an endpoint SHOULD send an ACK
 frame immediately on receiving an ack-eliciting packet that is out of order. The
-endpoint MAY continue sending ACK frames immediately on each subsequently
-received packet, but the endpoint SHOULD return to acknowledging every other
-packet within a period of 1/8 x RTT, unless more ack-eliciting packets are
-received out of order.  If every subsequent ack-eliciting packet arrives out of
-order, then an ACK frame SHOULD be sent immediately for every received
-ack-eliciting packet.
+endpoint MUST not continue sending ACK frames immediately unless more
+ack-eliciting packets are received out of order.  If every subsequent
+ack-eliciting packet arrives out of order, then an ACK frame SHOULD be sent
+immediately for every received ack-eliciting packet.
 
 Similarly, packets marked with the ECN Congestion Experienced (CE) codepoint in
 the IP header SHOULD be acknowledged immediately, to reduce the peer's response