Bugfixes

• Clarify timeout unit in KafkaProducer close and flush (ms7s PR 734)
• Avoid busy poll during metadata refresh failure with retry_backoff_ms (dpkp PR 733)
• Check_version should scan nodes until version found or timeout (dpkp PR 731)
• Fix bug which could cause least_loaded_node to always return the same unavailable node (dpkp PR 730)
• Fix producer garbage collection with weakref in atexit handler (dpkp PR 728)
• Close client selector to fix fd leak (msmith PR 729)
• Tweak spelling mistake in error const (steve8918 PR 719)
• Rearrange connection tests to separate legacy KafkaConnection