simplify generation of STREAM_DATA_BLOCKED frames (#4608)

The stream always gets blocked at the current write offset. There's no
need to return this offset from the flow controller.

internal/flowcontrol/interface.go

@@ -9,7 +9,6 @@ type flowController interface {
 	AddBytesSent(protocol.ByteCount)
 	// for receiving
 	GetWindowUpdate() protocol.ByteCount // returns 0 if no update is necessary
-	IsNewlyBlocked() (bool, protocol.ByteCount)
 }
 
 // A StreamFlowController is a flow controller for a QUIC stream.
@@ -23,6 +22,7 @@ type StreamFlowController interface {
 	// Abandon is called when reading from the stream is aborted early,
 	// and there won't be any further calls to AddBytesRead.
 	Abandon()
+	IsNewlyBlocked() bool
 }
 
 // The ConnectionFlowController is the flow controller for the connection.
@@ -30,6 +30,7 @@ type ConnectionFlowController interface {
 	flowController
 	AddBytesRead(protocol.ByteCount)
 	Reset() error
+	IsNewlyBlocked() (bool, protocol.ByteCount)
 }
 
 type connectionFlowControllerI interface {