Version 1.6.0

• Adds Breaker.GetState() to check the breaker state directly (e.g. for monitoring metrics).
• Fix a race condition in the Batcher that could have lead to a panic if multiple batches ended up executing at once (thanks to Tiago Peczenyj for the discovery and very clear bug report).
• Fix Batcher.Shutdown() to behave correctly when multiple batches end up executing at once.
• A variety of small refactors, simplifications, and test suite improvements.