KAFKA-6622; Fix performance issue loading consumer offsets (#4661)

`batch.baseOffset` is an expensive operation (even says so in its javadoc), and yet was called for every single record in a batch when loading offsets. This means that for N records in a gzipped batch, the entire batch will be unzipped N times. The fix is to compute and cache the base offset once as we decompress and process the batch.

Reviewers: Dong Lin <lindong28@gmail.com>, Ismael Juma <ismael@juma.me.uk>, Jason Gustafson <jason@confluent.io>

core/src/main/scala/kafka/coordinator/group/GroupMetadataManager.scala

@@ -555,8 +555,11 @@ class GroupMetadataManager(brokerId: Int,
               }
               pendingOffsets.remove(batch.producerId)
             } else {
+              var batchBaseOffset: Option[Long] = None
               for (record <- batch.asScala) {
                 require(record.hasKey, "Group metadata/offset entry key should not be null")
+                if (batchBaseOffset.isEmpty)
+                  batchBaseOffset = Some(record.offset)
                 GroupMetadataManager.readMessageKey(record.key) match {
 
                   case offsetKey: OffsetKey =>
@@ -573,9 +576,9 @@ class GroupMetadataManager(brokerId: Int,
                     } else {
                       val offsetAndMetadata = GroupMetadataManager.readOffsetMessageValue(record.value)
                       if (isTxnOffsetCommit)
-                        pendingOffsets(batch.producerId).put(groupTopicPartition, CommitRecordMetadataAndOffset(Some(batch.baseOffset), offsetAndMetadata))
+                        pendingOffsets(batch.producerId).put(groupTopicPartition, CommitRecordMetadataAndOffset(batchBaseOffset, offsetAndMetadata))
                       else
-                        loadedOffsets.put(groupTopicPartition, CommitRecordMetadataAndOffset(Some(batch.baseOffset), offsetAndMetadata))
+                        loadedOffsets.put(groupTopicPartition, CommitRecordMetadataAndOffset(batchBaseOffset, offsetAndMetadata))
                     }
 
                   case groupMetadataKey: GroupMetadataKey =>