Change inserts to upserts in mongoclient

When running workloads that use the "insert" operation, we get a lot of
duplicate key exceptions from the server. Changing to upsert to avoid
these exceptions.

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

@@ -245,11 +245,16 @@ public class AsyncMongoDbClient extends DB {
             final MongoCollection collection = db.getCollection(table);
             final DocumentBuilder r = DOCUMENT_BUILDER.get().reset()
                     .add("_id", key);
+            final Document q = r.build();
             for (final Map.Entry<String, ByteIterator> entry : values
                     .entrySet()) {
                 r.add(entry.getKey(), entry.getValue().toArray());
             }
             collection.insert(writeConcern, r);
+
+            collection.update(q, r, /* multi= */false, /* upsert= */true,
+                    writeConcern);
+
             return 0;
         }
         catch (final Exception e) {

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

@@ -261,11 +261,15 @@ public class MongoDbClient extends DB {
             db.requestStart();
 
             DBCollection collection = db.getCollection(table);
-            DBObject r = new BasicDBObject().append("_id", key);
+            DBObject criteria = new BasicDBObject().append("_id", key);
+            DBObject toInsert = new BasicDBObject().append("_id", key);
+
             for (Map.Entry<String, ByteIterator> entry : values.entrySet()) {
-                r.put(entry.getKey(), entry.getValue().toArray());
+                toInsert.put(entry.getKey(), entry.getValue().toArray());
             }
-            collection.insert(r, writeConcern);
+
+            collection.update(criteria, toInsert, true, false, writeConcern);
+
             return 0;
         }
         catch (Exception e) {