internalize ackhandler and congestion

internal/congestion/stats.go

@@ -0,0 +1,8 @@
+package congestion
+
+import "github.com/lucas-clemente/quic-go/internal/protocol"
+
+type connectionStats struct {
+	slowstartPacketsLost protocol.PacketNumber
+	slowstartBytesLost   protocol.ByteCount
+}