Code factored out of
  https://github.com/wikimedia/change-propagation/blob/06178c20779c5ff213dcfdaca92fef0072e0d469/lib/kafka_factory.js
and
  https://github.com/wikimedia/eventgate/blob/892079f71ebce0499312b9bf09738577186b5ff6/lib/kafka.js

Exported API modified to use .factory() method on exported classes instead of a separate
producerFactory() function.  This allows users to instantiate the classes via the class
factory method, rather than using a mapping of string producer type to ProducerClass.

Bug: T266058
Change-Id: If4119c3b822d07e5bc6086b8d408264a2a7b3e95