Bugfixes

• Fix partition assignment race condition (jeffwidman #1240)
• Fix consumer bug when seeking / resetting to the middle of a compressed messageset (dpkp #1239)
• Fix traceback sent to stderr not logging (dbgasaway #1221)
• Stop using mutable types for default arg values (jeffwidman #1213)
• Remove a few unused imports (jameslamb #1188)

Client

• Refactor BrokerConnection to use asynchronous receive_bytes pipe (dpkp #1032)

Consumer

• Drop unused sleep kwarg to poll (dpkp #1177)
• Enable KafkaConsumer beginning_offsets() and end_offsets() with older broker versions (buptljy #1200)
• Validate consumer subscription topic strings (nikeee #1238)

Documentation

• Small fixes to SASL documentation and logging; validate security_protocol (dpkp #1231)
• Various typo and grammar fixes (jeffwidman)