- add unit test to reproduce the deadlock by simulating a network error
- document possible deadlock when closing the Broker from an AsyncProduce
  callback when handling a response error
- add closeBroker goroutine and channel to asynchronously close a Broker
  once
- reuse the stopchan channel to signal that the closeBroker goroutine is
  done
- update TestBrokerProducerShutdown to check goroutine leak by closing
  the input vs the stopchan channel
- fixes IBM#2129

WARNING: DATA RACE
Write at 0x00c0003421f0 by goroutine 71:
  runtime.closechan()
      runtime/chan.go:355 +0x0
  github.com/Shopify/sarama.(*asyncProducer).newBrokerProducer.func1()
      github.com/Shopify/sarama/async_producer.go:725 +0x1c4
  github.com/Shopify/sarama.withRecover()
      github.com/Shopify/sarama/utils.go:43 +0x74
  github.com/Shopify/sarama.(*asyncProducer).newBrokerProducer·dwrap·15()
      github.com/Shopify/sarama/async_producer.go:695 +0x39

Previous read at 0x00c0003421f0 by goroutine 58:
  runtime.chansend()
      runtime/chan.go:158 +0x0
  github.com/Shopify/sarama.(*asyncProducer).newBrokerProducer.func1.1.1()
      github.com/Shopify/sarama/async_producer.go:702 +0x125
  github.com/Shopify/sarama.(*Broker).AsyncProduce.func1()
      github.com/Shopify/sarama/broker.go:408 +0x1a9
  github.com/Shopify/sarama.(*responsePromise).handle()
      github.com/Shopify/sarama/broker.go:132 +0x1b8
  github.com/Shopify/sarama.(*Broker).responseReceiver()
      github.com/Shopify/sarama/broker.go:1040 +0x124
  github.com/Shopify/sarama.(*Broker).responseReceiver-fm()
      github.com/Shopify/sarama/broker.go:1032 +0x39
  github.com/Shopify/sarama.withRecover()
      github.com/Shopify/sarama/utils.go:43 +0x74
  github.com/Shopify/sarama.(*Broker).Open.func1·dwrap·22()
      github.com/Shopify/sarama/broker.go:244 +0x39

Closing the broker asynchronously fixes the deadlock but leads to a
race condition between opening the broker in client updateLeader.
This might result in a closed broker used by the new brokerProducer
and all produce requests will fail with ErrNotConnected.