- Cluster: In FetchOffsets if there is an error, try refreshing metadata between retries on fetching offsets

- Producer: change the way messages are buffered for sending to use a single circular buffer of messages for each partition.
- Producer: Introduce new configuration settings AutoGrowSendBuffers, SendBuffersInitialSize, and MaxMessagesPerProduceRequest
- Tests: new test for AutoGrowSendBuffers, add close for some missed resources at the end of test, fix some other misc stuff in tests