DistributedExecutorMassIndexer.executeInternal should use an existing pool

DistributedExecutorMassIndexer.executeInternal creates 2 executors:

1. new DefaultExecutorService(cache) implicitly creates a single-threaded executor to run tasks on the local node.
2. compositeFuture.whenCompleteAsync(consumer, Executors.newSingleThreadExecutor()) explicitly creates a new single-threaded executor to run the consumer on.

Neither of these executors are shut down, and rather than complicating the code to stop them properly it would be better to use an existing executor.