Don't use finalizers to close cursors

mongodb/src/main/java/com/yahoo/ycsb/db/MongoDbClient.java

@@ -135,6 +135,7 @@ public class MongoDbClient extends DB {
                 System.out.println("new database url = " + url);
                 MongoClientOptions options = MongoClientOptions
                         .builder()
+                        .cursorFinalizerEnabled(false)
                         .connectionsPerHost(Integer.parseInt(maxConnections))
                         .threadsAllowedToBlockForConnectionMultiplier(
                                 Integer.parseInt(threadsAllowedToBlockForConnectionMultiplier))
@@ -373,6 +374,7 @@ public class MongoDbClient extends DB {
     public int scan(String table, String startkey, int recordcount,
             Set<String> fields, Vector<HashMap<String, ByteIterator>> result) {
         com.mongodb.DB db = null;
+        DBCursor cursor = null;
         try {
             db = mongo.getDB(database);
             db.requestStart();
@@ -380,7 +382,7 @@ public class MongoDbClient extends DB {
             // { "_id":{"$gte":startKey, "$lte":{"appId":key+"\uFFFF"}} }
             DBObject scanRange = new BasicDBObject().append("$gte", startkey);
             DBObject q = new BasicDBObject().append("_id", scanRange);
-            DBCursor cursor = collection.find(q).limit(recordcount);
+            cursor = collection.find(q).limit(recordcount);
             while (cursor.hasNext()) {
                 // toMap() returns a Map, but result.add() expects a
                 // Map<String,String>. Hence, the suppress warnings.
@@ -400,6 +402,9 @@ public class MongoDbClient extends DB {
         }
         finally {
             if (db != null) {
+                if (cursor != null) {
+                    cursor.close();
+                }
                 db.requestDone();
             }
         }