fix stalls when retransmitting connection-level Window Update Frames

When a Window Update Frame with streamID 0 is lost, we want to know
the receive window when retransmitting the frame. However, the connection
flow controller is not in streamFlowController, leading to map access errors
and then connections stalls since the sender is stuck by the not-updated
receive window it previously received.

flowcontrol/flow_control_manager.go

@@ -157,6 +157,11 @@ func (f *flowControlManager) GetReceiveWindow(streamID protocol.StreamID) (proto
 	f.mutex.RLock()
 	defer f.mutex.RUnlock()
 
+	// StreamID can be 0 when retransmitting
+	if streamID == 0 {
+		return f.connFlowController.receiveWindow, nil
+	}
+
 	flowController, err := f.getFlowController(streamID)
 	if err != nil {
 		return 0, err

flowcontrol/flow_control_manager_test.go

@@ -103,6 +103,12 @@ var _ = Describe("Flow Control Manager", func() {
 			Expect(err).To(MatchError(errMapAccess))
 		})
 
+		It("gets the offset of the connection-level receive window", func() {
+			offset, err := fcm.GetReceiveWindow(0)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(offset).To(Equal(protocol.ByteCount(200)))
+		})
+
 		Context("flow control violations", func() {
 			It("errors when encountering a stream level flow control violation", func() {
 				err := fcm.UpdateHighestReceived(4, 101)