fix(spanner): unflake unit tests

Author: sakthivelmanii

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

@@ -65,10 +65,11 @@ abstract class ResumableStreamIterator extends AbstractIterator<PartialResultSet
   private static final Logger logger = Logger.getLogger(ResumableStreamIterator.class.getName());
   private BackOff backOff;
   private final LinkedList<PartialResultSet> buffer = new LinkedList<>();
+  private final Object monitor = new Object();
   private final int maxBufferSize;
   private final ISpan span;
   private final TraceWrapper tracer;
-  private CloseableIterator<PartialResultSet> stream;
+  private volatile CloseableIterator<PartialResultSet> stream;
   private ByteString resumeToken;
   private boolean finished;
   /**