Process all topic/partitions even if one has an error

Otherwise an error in one partition we don't care about might prevent us from
processing the partitions we *do* care about.

One small part of #210.
Merge pull request #211 from Shopify/process-all-partitions

Process all topic/partitions even if one has an error
Permit real concurrent calls to the SimpleProducer

Without using a mutex.
Rename internal variable
Add test for concurrent simple-producer
producer: ensure returned messages have no flags

Otherwise, feeding a returned message back into the input could confuse the
state machine.
Merge pull request #214 from Shopify/retryable-producer

producer: ensure returned messages have no flags
Merge pull request #212 from Shopify/performant-simple-producer

Permit real concurrent calls to the SimpleProducer
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
WIP