producer: check MaxMessagesPerReq >= FlushMsgCount

Validate the above condition when MaxMessagesPerReq is set, as it doesn't make
sense to force a flush before one has been soft-triggered.
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Merge pull request #240 from Shopify/tighten-config-validation

producer: check MaxMessagesPerReq >= FlushMsgCount
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