WritePrepared Txn: disable rollback in stress test

Summary:
WritePrepared rollback implementation is not ready to be invoked in the middle of workload. This is due the lack of synchronization to obtain the cf handle from db. Temporarily disabling this until the problem with rollback is fixed.
Closes https://github.com/facebook/rocksdb/pull/3772

Differential Revision: D7769041

Pulled By: maysamyabandeh

fbshipit-source-id: 0e3b0ce679bc2afba82e653a40afa3f045722754

util/transaction_test_util.cc

@@ -184,7 +184,9 @@ bool RandomTransactionInserter::DoInsert(DB* db, Transaction* txn,
         s = txn->Prepare();
         assert(s.ok());
       }
-      if (!rand_->OneIn(20)) {
+      // TODO(myabandeh): enable this when WritePreparedTxnDB::RollbackPrepared
+      // is updated to handle in-the-middle rollbacks.
+      if (!rand_->OneIn(0)) {
         s = txn->Commit();
       } else {
         // Also try 5% rollback