ISSUE-448: Access to partitionConsumer state not thread-safe.

[sijie]

Original Issue: apache#448

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As part of my testing of the fix for apache#432, I have discovered a race condition in consumer_partition.go where access to the struct's state member is modified concurrently without synchronization:

WARNING: DATA RACE
Read at 0x00c00039a918 by goroutine 9:
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).clearQueueAndGetNextMessage()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:911 +0x68
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).clearReceiverQueue()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:932 +0x84
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).grabConn()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:861 +0x1312
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).reconnectToBroker()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:802 +0x238
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).runEventsLoop.func2()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:711 +0xab

Previous write at 0x00c00039a918 by goroutine 70:
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).internalClose()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:772 +0x68f
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).runEventsLoop()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:732 +0x29b

Goroutine 9 (running) created at:
  github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).runEventsLoop()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:704 +0x146

Goroutine 70 (finished) created at:
  github.com/apache/pulsar-client-go/pulsar.newPartitionConsumer()
      /Users/me/pulsar-client-go/pulsar/consumer_partition.go:184 +0xead
  github.com/apache/pulsar-client-go/pulsar.(*consumer).internalTopicSubscribeToPartitions.func1()
      /Users/me/pulsar-client-go/pulsar/consumer_impl.go:314 +0x7d3