fix: protect writes with monitor

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/AsyncResultSetImpl.java

@@ -498,10 +498,12 @@ public ApiFuture<Void> setCallback(Executor exec, ReadyCallback cb) {
   }
 
   private void initiateProduceRows() {
-    if (this.state == State.STREAMING_INITIALIZED) {
-      this.state = State.RUNNING;
+    synchronized (monitor) {
+      if (this.state == State.STREAMING_INITIALIZED) {
+        this.state = State.RUNNING;
+      }
+      produceRowsInitiated = true;
     }
-    produceRowsInitiated = true;
     this.service.execute(new ProduceRowsRunnable());
   }