Related to #42
Adds new ConsumerFetchSizeTooSmall exception that is thrown when
`_decode_message_set_iter` gets a BufferUnderflowError but has not yet
yielded a message
In this event, SimpleConsumer will increase the fetch size by 1.5 and
continue the fetching loop while _not_ increasing the offset (basically
just retries the request with a larger fetch size)
Once the consumer fetch size has been increased, it will remain
increased while SimpleConsumer fetches from that partition
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