tweaks to address code review comments
Make maximum-parsable response size configurable

Replace hard-coded maxMessageSize with a configurable MaxResponseSize to match
the already-configurable MaxRequestSize.

Fixes #216.
Merge pull request #217 from Shopify/configurable-max-response

Make maximum-parsable response size configurable
Merge pull request #215 from Shopify/consistent-partitioning

Consistent partitioning
producer: wrap leader updates in a circuit-breaker

Otherwise a partition being entirely gone can slow things right down, since
we'll do a complete metadata refresh on every single message.