Address a data race on numMessages in DirectStreamObserver.java

[t2h6]

Fixes a data race found in DirectStreamObserver.java (between onNext() and <init>):

WARNING: ThreadSanitizer: data race (pid=9059)
  Read of size 4 at 0x00009409f010 by thread T3381 (mutexes: write M0):
    #0 org.apache.beam.sdk.fn.stream.DirectStreamObserver.onNext(Ljava/lang/Object;)V DirectStreamObserver.java:72 
    #1 org.apache.beam.fn.harness.control.BeamFnControlClient.sendInstructionResponse(Lorg/apache/beam/model/fnexecution/v1/BeamFnApi$InstructionResponse;)V BeamFnControlClient.java:175 
    #2 org.apache.beam.fn.harness.control.BeamFnControlClient$InboundObserver.lambda$onNext$0(Lorg/apache/beam/model/fnexecution/v1/BeamFnApi$InstructionRequest;)V BeamFnControlClient.java:116 
    #3 org.apache.beam.fn.harness.control.BeamFnControlClient$InboundObserver$$Lambda$562.run()V ?? 
    #4 java.util.concurrent.Executors$RunnableAdapter.call()Ljava/lang/Object; Executors.java:515 
    #5 java.util.concurrent.FutureTask.run()V FutureTask.java:264 
    #6 org.apache.beam.sdk.util.UnboundedScheduledExecutorService$ScheduledFutureTask.run()V UnboundedScheduledExecutorService.java:163 
    #7 java.util.concurrent.ThreadPoolExecutor.runWorker(Ljava/util/concurrent/ThreadPoolExecutor$Worker;)V ThreadPoolExecutor.java:1130 
    #8 java.util.concurrent.ThreadPoolExecutor$Worker.run()V ThreadPoolExecutor.java:630 
    #9 java.lang.Thread.run()V Thread.java:830 
    #10 (Generated Stub) <null> 

  Previous write of size 4 at 0x00009409f010 by thread T3293:
    #0 org.apache.beam.sdk.fn.stream.DirectStreamObserver.<init>(Ljava/util/concurrent/Phaser;Lio/grpc/stub/CallStreamObserver;I)V DirectStreamObserver.java:56 
    #1 org.apache.beam.sdk.fn.stream.DirectStreamObserver.<init>(Ljava/util/concurrent/Phaser;Lio/grpc/stub/CallStreamObserver;)V DirectStreamObserver.java:59 
    #2 org.apache.beam.sdk.fn.stream.OutboundObserverFactory$DirectClient.outboundObserverFor(Lorg/apache/beam/sdk/fn/stream/OutboundObserverFactory$BasicFactory;Lio/grpc/stub/StreamObserver;)Lio/grpc/stub/StreamObserver; OutboundObserverFactory.java:98 
    #3 org.apache.beam.fn.harness.control.BeamFnControlClient.<init>(Lorg/apache/beam/model/fnexecution/v1/BeamFnControlGrpc$BeamFnControlStub;Lorg/apache/beam/sdk/fn/stream/OutboundObserverFactory;Ljava/util/concurrent/Executor;Ljava/util/EnumMap;)V BeamFnControlClient.java:87 
    #4 org.apache.beam.fn.harness.FnHarness.main(Ljava/lang/String;Lorg/apache/beam/sdk/options/PipelineOptions;Ljava/util/Set;Lorg/apache/beam/model/pipeline/v1/Endpoints$ApiServiceDescriptor;Lorg/apache/beam/model/pipeline/v1/Endpoints$ApiServiceDescriptor;Lor FnHarness.java:409 

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[kennknowles]

I'm not wholly convinced this result is legitimate. It seems to be saying that the objects construction is in a data race with a callback on that object.

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