NPE during transaction collisions

Found the following exception thrown many times in logs during a failed stress test where 100K integers were loaded and count at root node was expected to be 99,999 but was instead 100,462.

```
20:18:12.828 [pool-19-thread-3] DEBUG io.fluo.core.impl.Worker - Failure processing notification concurrently
java.lang.NullPointerException: null
at io.fluo.core.impl.TransactionImpl.checkForAckCollision(TransactionImpl.java:523) ~[fluo-core-1.0.0-alpha-1.jar:na]
at io.fluo.core.impl.TransactionImpl.preCommit(TransactionImpl.java:393) ~[fluo-core-1.0.0-alpha-1.jar:na]
at io.fluo.core.impl.TransactionImpl.preCommit(TransactionImpl.java:340) ~[fluo-core-1.0.0-alpha-1.jar:na]
at io.fluo.core.impl.TransactionImpl.commit(TransactionImpl.java:664) ~[fluo-core-1.0.0-alpha-1.jar:na]
at io.fluo.core.impl.Worker.processUpdates(Worker.java:180) ~[fluo-core-1.0.0-alpha-1.jar:na]
at io.fluo.core.impl.WorkerTask.run(WorkerTask.java:65) [fluo-core-1.0.0-alpha-1.jar:na]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [na:1.7.0_51]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_51]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_51]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_51]
at java.lang.Thread.run(Thread.java:744) [na:1.7.0_51]
```