producer: permit throttling max messages per request

Related to #233
producer: clarify documentation around Flush*
Merge pull request #235 from Shopify/max-messages-per-request

producer: permit throttling max messages per request
Move retries to its own field and make MaxRetries configurable

Doesn't work yet, but these are the superficial changes necessary to support
multiple retries in the producer.
producer: support more than one in-order retry

See #232
Permit configuring MaxRetries > 1