Producer Shutdown Testing: Graceful Shutdown

[markglh]

This should be tackled AFTER: #6 & #7

Applications using the Kinesis client should gracefully shutdown. By this we mean completing the current batch (both in the actor and in the KPL), to minimise duplication of messages.

The Producer doesn't currently do anything special here, but it should.

This must be fully tested by asserting duplication of messages published to a stream over a number of shutdowns

The outcome here is to verify how much duplication of messages happens under normal an exception scenarios. Exception meaning adding/removing shards, shutting down a consumer/producer, etc. Document the outcome and raise any necessary stories required to address unacceptable duplication.