move congestion clock to new file and implement default clock

congestion/clock.go

@@ -0,0 +1,18 @@
+package congestion
+
+import "time"
+
+// A Clock returns the current time
+type Clock interface {
+	Now() time.Time
+}
+
+// DefaultClock implements the Clock interface using the Go stdlib clock.
+type DefaultClock struct{}
+
+var _ Clock = DefaultClock{}
+
+// Now gets the current time
+func (DefaultClock) Now() time.Time {
+	return time.Now()
+}

congestion/cubic.go

@@ -7,11 +7,6 @@ import (
 	"github.com/lucas-clemente/quic-go/protocol"
 )
 
-// A Clock returns the current time
-type Clock interface {
-	Now() time.Time
-}
-
 // This cubic implementation is based on the one found in Chromiums's QUIC
 // implementation, in the files net/quic/congestion_control/cubic.{hh,cc}.