producer_worker: revert use of ValueGenerator::Generate()

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

```
Benchmark_random_payload10-32    2445903       485.2 ns/op
Benchmark_random_payload100-32    678870        1773 ns/op
Benchmark_random_payload1000-32   105667       13700 ns/op
Benchmark_random_payload1e4-32      9104      126782 ns/op
Benchmark_random_payload1e5-32      1135     1151425 ns/op
Benchmark_random_payload1e6-32       100    13046547 ns/op
==========================================================
Benchmark_empty_payload10-32    79950942       20.54 ns/op
Benchmark_empty_payload100-32   24089101       54.78 ns/op
Benchmark_empty_payload1000-32   3789248       366.1 ns/op
Benchmark_empty_payload1e4-32     670096        2059 ns/op
Benchmark_empty_payload1e5-32      58912       23021 ns/op
Benchmark_empty_payload1e6-32       5600      290045 ns/op
```

As can be seen, generating the random payload and ensuring it is UTF-8 valid
is orders of magnitude slower than the empty payload.

Revert the use of `Generate()` here in place of the empty payload.

However, if the user has indicated they want to validate the latest key-value
pair produced, generate a `(value-{%018d}, offset)` message in the record.

This does mean that message sizes that are less than 24 bytes are not honored
if the `validate-latest-values` flag is passed.

pkg/worker/verifier/producer_worker.go

@@ -121,10 +121,25 @@ 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 {
+		if pw.validateLatestValues {
+			var value bytes.Buffer
+			fmt.Fprintf(&value, "value-%018d", sequence)
+			paddingSize := pw.config.messageSize - value.Len()
+			if paddingSize > 0 {
+				value.Write(make([]byte, paddingSize))
+			}
+			payload = value.Bytes()
+		} else {
+			payload = make([]byte, pw.config.messageSize)
+		}
 	}
+
 	var r *kgo.Record
 
 	if pw.config.keySetCardinality < 0 {

pkg/worker/worker.go

@@ -108,10 +108,6 @@ type ValueGenerator struct {
 var compressible_payload []byte
 
 func (vg *ValueGenerator) Generate() []byte {
-	isTombstone := rand.Float64() < vg.TombstoneProbability
-	if isTombstone {
-		return nil
-	}
 	if vg.Compressible {
 		// Zeros, which is about as compressible as an array can be.
 		if len(compressible_payload) == 0 {