Add a mutex to SimpleProducer.SendMessage

The dumbest (and least performant) way to make SimpleProducer concurrency-safe.
The quick fix for the most concerning part of #187, since the proper concurrency
fix won't have any effect until we can do multiple requests in flight again
anyways.
Merge pull request #192 from Shopify/lock-simple-producer

Add a mutex to SimpleProducer.SendMessage