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.

stream_test.go

@@ -140,7 +140,10 @@ var _ = Describe("Stream", func() {
 					Expect(err).ToNot(HaveOccurred())
 					close(writeReturned)
 				}()
-				Eventually(func() *wire.StreamFrame { return str.popStreamFrame(1000) }).ShouldNot(BeNil())
+				Eventually(func() *wire.StreamFrame {
+					frame, _ := str.popStreamFrame(1000)
+					return frame
+				}).ShouldNot(BeNil())
 				Eventually(writeReturned).Should(BeClosed())
 				mockSender.EXPECT().queueControlFrame(&wire.RstStreamFrame{
 					StreamID:   streamID,