Batch Scanner can get stuck when external thread closes scanner

Scenario- client is using batch scanner to run queries with complex server side iterators. They run long. User has external thread which determines thread is running long, so that thread calls close on the batch scanner. Query threads exit nicely, but there is a thread which is constantly polling an internal queue. It only breaks out of that loop if there is a next key or an error thrown. If neither occur, it continues to poll. We should probably have an exit condition get set on close() which is used in that query loop.