Enhancements and new features

• Avro Serializer and Deserializer
• Various performance improvements
• Moved to librdkafka v0.11.4

Fixes

• handle OperationCanceledException (#446)
• added single partition overloads for Assign (#417)
• Optimize topic handle lookup (#401)
• Structs and strings marshaling performance optimization (#403)
• use ThrowIfCancellationRequested to exit producer poll loop (#413)
• set produce.offset.report to true by default (#410)
• Load librdkafka automatically on Library.Version etc. (#392)