cons: Don't execute Poll & shutdown concurrently

[agis]

In librdkafka/confluent-kafka-go we shouldn't call Poll() concurrently with Close() or Unsubscribe(), since that might lead to race conditions. We noticed such an issue when upgrading to librdkafka 0.11.6 and 1.0.0, where consumers would block in Poll() forever if rafka attempted to shutdown right in the middle of a call to Poll() (i.e. clients calling BLPOP). See edenhill/librdkafka#2333 for background.

This patch synchronizes access between a consumer's Poll() and its shutdown sequence, so that the two may never run concurrently. Furthermore, if a consumer is terminated then calling Poll() will immediately return nil without communicating with Kafka at all.

Fixes consumers blocking on Poll() when the server is shutting down, with librdkafka 0.11.6 and 1.0.0.

[avgerin0s]

Good job @agis

[konstantinoskostis]

Nice @agis! Great job!