producer_worker: revert use of ValueGenerator::Generate()

Using the modified `Generate()` function was a lot slower than
just simply making an empty payload.

Author: WillemKauf

pkg/worker/verifier/producer_worker.go

@@ -121,10 +121,15 @@ func (pw *ProducerWorker) newRecord(producerId int, sequence int64) *kgo.Record
 		pw.Status.AbortedTransactionMessages += 1
 	}
 
-	payload := pw.config.valueGenerator.Generate()
-	if payload == nil {
+	var payload []byte
+	isTombstone := rand.Float64() < pw.config.valueGenerator.TombstoneProbability
+	if isTombstone {
+		payload = nil
 		pw.Status.TombstonesProduced += 1
+	} else {
+		payload = make([]byte, pw.config.messageSize)
 	}
+
 	var r *kgo.Record
 
 	if pw.config.keySetCardinality < 0 {