move the RTTStats to the utils package

The RTTStats are used by the logging package. In order to instrument the
congestion package, the RTTStats can't be part of that package any more
(to avoid an import loop).

internal/flowcontrol/base_flow_controller.go

@@ -4,7 +4,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/lucas-clemente/quic-go/internal/congestion"
 	"github.com/lucas-clemente/quic-go/internal/protocol"
 	"github.com/lucas-clemente/quic-go/internal/utils"
 )
@@ -25,7 +24,7 @@ type baseFlowController struct {
 
 	epochStartTime   time.Time
 	epochStartOffset protocol.ByteCount
-	rttStats         *congestion.RTTStats
+	rttStats         *utils.RTTStats
 
 	logger utils.Logger
 }