Fix replay of requests with timestampp in "past"

In some rare cases, requests can be recorded out of order.
It was causing replay from file significanty slodown when such issue met.
Now it replays them but without adding sleep.

input_file.go

@@ -238,13 +238,15 @@ func (i *FileInput) emit() {
 
 		if lastTime != -1 {
 			diff := reader.timestamp - lastTime
-			lastTime = reader.timestamp
 
 			if i.speedFactor != 1 {
 				diff = int64(float64(diff) / i.speedFactor)
 			}
 
-			time.Sleep(time.Duration(diff))
+			if diff >= 0 {
+				lastTime = reader.timestamp
+				time.Sleep(time.Duration(diff))
+			}
 		} else {
 			lastTime = reader.timestamp
 		}