Fix db_stress failure (pass merge_operator even if not used)

Summary:
db_stress test is now failing because of this scenario
- run db_stress with merge_operator enabled (now we have a db with merge operands)
- run db_stress with merge_operator disabled (now when we fail to open the db)

the solution is to pass the merge_operator to the DB even if we are not going to do any merge operations

Test Plan: Check the failure

Reviewers: andrewkr, yiwu, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61311

tools/db_stress.cc

@@ -2109,12 +2109,10 @@ class StressTest {
 #endif  // ROCKSDB_LITE
     }
 
-    if (FLAGS_use_merge) {
-      if (FLAGS_use_full_merge_v1) {
-        options_.merge_operator = MergeOperators::CreateDeprecatedPutOperator();
-      } else {
-        options_.merge_operator = MergeOperators::CreatePutOperator();
-      }
+    if (FLAGS_use_full_merge_v1) {
+      options_.merge_operator = MergeOperators::CreateDeprecatedPutOperator();
+    } else {
+      options_.merge_operator = MergeOperators::CreatePutOperator();
     }
 
     // set universal style compaction configurations, if applicable