ackhandler: use a frame handler interface for OnAcked / OnLost of all frame types (#3888)

internal/ackhandler/frame.go

@@ -4,16 +4,18 @@ import (
 	"github.com/quic-go/quic-go/internal/wire"
 )
 
+// FrameHandler handles the acknowledgement and the loss of a frame.
+type FrameHandler interface {
+	OnAcked(wire.Frame)
+	OnLost(wire.Frame)
+}
+
 type Frame struct {
 	Frame   wire.Frame // nil if the frame has already been acknowledged in another packet
-	OnLost  func(wire.Frame)
-	OnAcked func(wire.Frame)
+	Handler FrameHandler
 }
 
 type StreamFrame struct {
 	Frame   *wire.StreamFrame
-	Handler interface {
-		OnLost(*wire.StreamFrame)
-		OnAcked(*wire.StreamFrame)
-	}
+	Handler FrameHandler
 }