treat committed offset as next fetch, not last

this brings the behavior of the balanced consumer in line with the java implementation

Author: ducas

src/KafkaNET.Library/Consumers/ZookeeperConsumerConnector.cs

@@ -142,7 +142,7 @@ public void CommitOffsets()
                     var topicDirs = new ZKGroupTopicDirs(this.config.GroupId, topic.Key);
                     foreach (KeyValuePair<int, PartitionTopicInfo> partition in topic.Value)
                     {
-                        var newOffset = partition.Value.ConsumeOffset;
+                        var newOffset = partition.Value.ConsumeOffset + 1;
                         try
                         {
                             if (partition.Value.ConsumeOffsetValid)

src/KafkaNET.Library/ZooKeeperIntegration/Listeners/ZKRebalancerListener.cs

@@ -501,26 +501,24 @@ private void AddPartitionTopicInfo(ZKGroupTopicDirs topicDirs, string partition,
 
             //if first time starting a consumer, set the initial offset based on the config
             long offset = -1;
-            long offsetCommited = -1;
             if (offsetCommitedString != null)
             {
-                offsetCommited = long.Parse(offsetCommitedString);
-                offset = offsetCommited + 1;
+                offset = long.Parse(offsetCommitedString);
             }
-            Logger.InfoFormat("Final offset {0} for topic {1} partition {2} OffsetCommited {3}"
-                    , offset, topic, partition, offsetCommited);
+            Logger.InfoFormat("Final offset {0} for topic {1} partition {2}."
+                    , offset, topic, partition);
 
             var queue = this.queues[new Tuple<string, string>(topic, consumerThreadId)];
             var partTopicInfo = new PartitionTopicInfo(
                 topic,
                 leader,
                 partitionId,
                 queue,
-                offsetCommited,
+                offset,
                 offset,
                 offset,
                 this.config.FetchSize,
-                offsetCommited);
+                offset);
             partTopicInfoMap[partitionId] = partTopicInfo;
             Logger.InfoFormat("{0} selected new offset {1}", partTopicInfo, offset);
         }