introduce a queue of active streams in the streamFramer

The queue holds all streams that have data to send. When stream.Write or
stream.Close are called, a stream is added to this queue. It is removed
from the queue when all available (at that moment) data was sent.
This way, we don't need the round robin scheduling (which, for every
packet sent, asked every single open stream if it had data) any more.

send_stream.go

@@ -122,12 +122,12 @@ func (s *sendStream) Write(p []byte) (int, error) {
 
 // popStreamFrame returns the next STREAM frame that is supposed to be sent on this stream
 // maxBytes is the maximum length this frame (including frame header) will have.
-func (s *sendStream) popStreamFrame(maxBytes protocol.ByteCount) *wire.StreamFrame {
+func (s *sendStream) popStreamFrame(maxBytes protocol.ByteCount) (*wire.StreamFrame, bool /* has more data to send */) {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
 	if s.closeForShutdownErr != nil {
-		return nil
+		return nil, false
 	}
 
 	frame := &wire.StreamFrame{
@@ -137,11 +137,11 @@ func (s *sendStream) popStreamFrame(maxBytes protocol.ByteCount) *wire.StreamFra
 	}
 	frameLen := frame.MinLength(s.version)
 	if frameLen >= maxBytes { // a STREAM frame must have at least one byte of data
-		return nil
+		return nil, s.dataForWriting != nil
 	}
 	frame.Data, frame.FinBit = s.getDataForWriting(maxBytes - frameLen)
 	if len(frame.Data) == 0 && !frame.FinBit {
-		return nil
+		return nil, s.dataForWriting != nil
 	}
 	if frame.FinBit {
 		s.finSent = true
@@ -153,7 +153,7 @@ func (s *sendStream) popStreamFrame(maxBytes protocol.ByteCount) *wire.StreamFra
 			})
 		}
 	}
-	return frame
+	return frame, s.dataForWriting != nil
 }
 
 func (s *sendStream) getDataForWriting(maxBytes protocol.ByteCount) ([]byte, bool /* should send FIN */) {