add cubic_sender plus initial test

congestion/interface.go

@@ -0,0 +1,13 @@
+package congestion
+
+import (
+	"time"
+
+	"github.com/lucas-clemente/quic-go/protocol"
+)
+
+type SendAlgorithm interface {
+	TimeUntilSend(now time.Time, bytesInFlight uint64) time.Duration
+	OnPacketSent(sentTime time.Time, bytesInFlight uint64, packetNumber protocol.PacketNumber, bytes uint64, isRetransmittable bool) bool
+	GetCongestionWindow() uint64
+}