queue connection-level window updates from the flow controller directly

It is not sufficient to check for connection-level window updates every
time a packet is sent. When a connection-level window update needs to be
sent, we need to make sure that it gets sent immediately (i.e. call
scheduleSending() in the session).

internal/flowcontrol/interface.go

@@ -10,6 +10,7 @@ type flowController interface {
 	// for receiving
 	AddBytesRead(protocol.ByteCount)
 	GetWindowUpdate() protocol.ByteCount // returns 0 if no update is necessary
+	MaybeQueueWindowUpdate()             //  queues a window update, if necessary
 }
 
 // A StreamFlowController is a flow controller for a QUIC stream.
@@ -21,8 +22,6 @@ type StreamFlowController interface {
 	// UpdateHighestReceived should be called when a new highest offset is received
 	// final has to be to true if this is the final offset of the stream, as contained in a STREAM frame with FIN bit, and the RST_STREAM frame
 	UpdateHighestReceived(offset protocol.ByteCount, final bool) error
-	// MaybeQueueWindowUpdate queues a window update, if necessary
-	MaybeQueueWindowUpdate()
 }
 
 // The ConnectionFlowController is the flow controller for the connection.