Summary:
Current db_stress does not cover complex read-write transactions. Therefore, this PR adds
coverage for emulated MyRocks-style transactions in `MultiOpsTxnsStressTest`. To achieve this, we need:
- Add a new operation type 'customops' so that we can add new complex groups of operations, e.g. transactions involving multiple read-write operations.
- Implement three read-write transactions and two read-only ones to emulate MyRocks-style transactions.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8936
Test Plan:
```
make check
./db_stress -test_multi_ops_txns -use_txn -clear_column_family_one_in=0 -column_families=1 -writepercent=0 -delpercent=0 -delrangepercent=0 -customopspercent=60 -readpercent=20 -prefixpercent=0 -iterpercent=20 -reopen=0 -ops_per_thread=100000
```
Next step is to add more configurability and refine input generation and result reporting, which will done in separate follow-up PRs.
Reviewed By: zhichao-cao
Differential Revision: D31071795
Pulled By: riversand963
fbshipit-source-id: 50d7c828346ec643311336b904848a1588a37006
