Avoid redundant killing of the cursor by setting a flag indicating that it's already been killed.

 JAVA-1854

Author: jyemin

driver-core/src/main/com/mongodb/operation/QueryBatchCursor.java

@@ -39,6 +39,7 @@ class QueryBatchCursor<T> implements BatchCursor<T> {
     private final ConnectionSource connectionSource;
     private int batchSize;
     private ServerCursor serverCursor;
+    private boolean killed;
     private List<T> nextBatch;
     private int count;
     private boolean closed;
@@ -228,8 +229,9 @@ private void killCursor() {
     }
 
     private void killCursor(final Connection connection) {
-        if (serverCursor != null) {
+        if (serverCursor != null && !killed) {
             connection.killCursor(asList(serverCursor.getId()));
+            killed = true;
         }
     }
 }