Update Rust crate rdkafka to 0.38.0

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This PR contains the following updates:

Package	Type	Update	Change
rdkafka	dependencies	minor	0.37.0 → 0.38.0

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Release Notes

fede1024/rust-rdkafka (rdkafka)

v0.38.0

• Update BaseProducer::poll to not return early, and instead continue
  looping until the passed timeout is reached.
• Breaking change. Change signature for OwnedDeliveryResult. The
  Ok variant is now a Delivery struct, rather than a tuple. This allows
  or including Timestamp as a result field. It means that adding values
  in the future will not require a breaking change.
• Update BaseProducer::flush to correctly call poll internally, until
  all messages have been processed.
• Upgrade all library dependencies.
• Add tests for Kafka versions up to 0.38.0.
• Require a minimum of CMake 3.5 compatibility for CMake build (#​766)

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