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

@@ -2109,13 +2109,11 @@ 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();
     }
-    }
 
     // set universal style compaction configurations, if applicable
     if (FLAGS_universal_size_ratio != 0) {