FluoIT triggers unexpected stack trace

The following "SEVERE" error showed up when running FluoIT. It looks like a task was attempted to be added to the Executor after it had terminated.

```
Running org.apache.fluo.integration.impl.FluoIT
Aug 04, 2016 3:39:51 PM com.google.common.util.concurrent.ExecutionList$RunnableExecutorPair execute
SEVERE: RuntimeException while executing runnable com.google.common.util.concurrent.Futures$6@74999c65 with executor org.apache.fluo.core.util.FluoExecutors$1@51993154[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 12]
java.util.concurrent.RejectedExecutionException: Task com.google.common.util.concurrent.Futures$6@74999c65 rejected from org.apache.fluo.core.util.FluoExecutors$1@51993154[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 12]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
at com.google.common.util.concurrent.ExecutionList$RunnableExecutorPair.execute(ExecutionList.java:149)
at com.google.common.util.concurrent.ExecutionList.execute(ExecutionList.java:134)
at com.google.common.util.concurrent.ListenableFutureTask.done(ListenableFutureTask.java:88)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:384)
at java.util.concurrent.FutureTask.set(FutureTask.java:233)
at java.util.concurrent.FutureTask.run(FutureTask.java:274)
at org.apache.fluo.core.impl.SharedBatchWriter$MutationBatch.countDown(SharedBatchWriter.java:74)
at org.apache.fluo.core.impl.SharedBatchWriter$FlushTask.processBatches(SharedBatchWriter.java:122)
at org.apache.fluo.core.impl.SharedBatchWriter$FlushTask.run(SharedBatchWriter.java:93)
at java.lang.Thread.run(Thread.java:745)

Tests run: 10, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 51.228 sec - in org.apache.fluo.integration.impl.FluoIT
```

Start by running the FluoIT integration test and inspect SharedBatchWriter.java, especially countDown and FlushTask.processBatches, along with the executor shown in FluoExecutors. Trace why completion handling reaches a terminated executor. Done means the test completes without the reported SEVERE RejectedExecutionException.

Written by the indexing model from the issue text.