Error when using PubsubIO with the DirectRunner

Reading a source with a `org.apache.beam.sdk.io.PubsubIO` and running the pipeline with `DirectRunner` result in an error.

The error occur usually after reading ~200 messages from pubsub at the moment where `PubsubIO` try to ack the messages.

The error:
```
Exception in thread "main" java.lang.IllegalStateException: Cannot finalize a restored checkpoint
at org.apache.beam.sdk.repackaged.com.google.common.base.Preconditions.checkState(Preconditions.java:174)
at org.apache.beam.sdk.io.PubsubUnboundedSource$PubsubCheckpoint.finalizeCheckpoint(PubsubUnboundedSource.java:293)
at org.apache.beam.runners.direct.UnboundedReadEvaluatorFactory$UnboundedReadEvaluator.finishRead(UnboundedReadEvaluatorFactory.java:205)
at org.apache.beam.runners.direct.UnboundedReadEvaluatorFactory$UnboundedReadEvaluator.processElement(UnboundedReadEvaluatorFactory.java:142)
at org.apache.beam.runners.direct.TransformExecutor.processElements(TransformExecutor.java:139)
at org.apache.beam.runners.direct.TransformExecutor.run(TransformExecutor.java:107)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
```