Assign relative offsets in compressed message sets

As previously noted (#903), Sarama currently doesn't assign inner
offsets in compressed messages. This means that the broker has to assign
relative offsets and then do recompression.

Which is fine! But if we assign offsets in the producer, then the broker can
skip recompression, improving broker CPU efficiency.

One way to see the effect is by adding some debug logging to Kafka with
the following patch:

```
diff --git a/core/src/main/scala/kafka/log/LogValidator.scala b/core/src/main/scala/kafka/log/LogValidator.scala
index 224a79277..4ecb98687 100644
--- a/core/src/main/scala/kafka/log/LogValidator.scala
+++ b/core/src/main/scala/kafka/log/LogValidator.scala
@@ -20,6 +20,7 @@ import java.nio.ByteBuffer

 import kafka.common.LongRef
 import kafka.message.{CompressionCodec, InvalidMessageException, NoCompressionCodec}
+import org.apache.log4j.Logger
 import org.apache.kafka.common.errors.InvalidTimestampException
 import org.apache.kafka.common.record._

@@ -27,6 +28,7 @@ import scala.collection.mutable
 import scala.collection.JavaConverters._

 private[kafka] object LogValidator {
+  private val log = Logger.getLogger(getClass())

   /**
    * Update the offsets for this message set and do further validation on messages including:
@@ -172,8 +174,13 @@ private[kafka] object LogValidator {
         // Validate the timestamp
         validateTimestamp(record, now, messageTimestampType, messageTimestampDiffMaxMs)
         // Check if we need to overwrite offset, no in place assignment situation 3
-        if (logEntry.offset != expectedInnerOffset.getAndIncrement())
+        val incrementedInnerOffset = expectedInnerOffset.getAndIncrement()
+        if (logEntry.offset != incrementedInnerOffset) {
+          log.info("overwriting inner offset; got " + logEntry.offset + " expected " + incrementedInnerOffset)
           inPlaceAssignment = false
+        } else {
+          log.info("no need to overwrite offset; got " + logEntry.offset + " expected " + incrementedInnerOffset)
+        }
         if (record.timestamp > maxTimestamp)
           maxTimestamp = record.timestamp
       }
```

This produces the following broker logs:

Before:
```
[2017-12-13 13:30:04,667] INFO no need to overwrite offset; got 0 expected 0 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 1 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 2 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 3 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 4 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 5 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 6 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 7 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 8 (kafka.log.LogValidator$)
[2017-12-13 13:30:01,465] INFO overwriting inner offset; got 0 expected 9 (kafka.log.LogValidator$)
```

After:
```
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 0 expected 0 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 1 expected 1 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 2 expected 2 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 3 expected 3 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 4 expected 4 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 5 expected 5 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 6 expected 6 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 7 expected 7 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 8 expected 8 (kafka.log.LogValidator$)
[2017-12-13 13:40:57,995] INFO no need to overwrite offset; got 9 expected 9 (kafka.log.LogValidator$)
```

produce_set.go

@@ -132,6 +132,11 @@ func (ps *produceSet) buildRequest() *ProduceRequest {
 				// and sent as the payload of a single fake "message" with the appropriate codec
 				// set and no key. When the server sees a message with a compression codec, it
 				// decompresses the payload and treats the result as its message set.
+				for i, msg := range set.recordsToSend.msgSet.Messages {
+					// Assign relative offsets to the inner messages. This lets
+					// the broker avoid recompressing the message set.
+					msg.Offset = int64(i)
+				}
 				payload, err := encode(set.recordsToSend.msgSet, ps.parent.conf.MetricRegistry)
 				if err != nil {
 					Logger.Println(err) // if this happens, it's basically our fault.

produce_set_test.go

@@ -173,11 +173,14 @@ func TestProduceSetCompressedRequestBuilding(t *testing.T) {
 		if err != nil {
 			t.Error("Failed to decode set from payload")
 		}
-		for _, compMsgBlock := range msg.Set.Messages {
+		for i, compMsgBlock := range msg.Set.Messages {
 			compMsg := compMsgBlock.Msg
 			if compMsg.Version != 1 {
 				t.Error("Wrong compressed message version")
 			}
+			if compMsgBlock.Offset != int64(i) {
+				t.Error("Wrong relative inner offset")
+			}
 		}
 		if msg.Version != 1 {
 			t.Error("Wrong compressed parent message version")