[YCSB] Change the implementation style of YCSB

[rotaki]

There are a few ways to implement YCSB in transactional systems.

1. Read, Update, RMW are executed separately on different transactions and the portion of each transaction is decided by the workload specification.
2. Read, Update, RMW are mixed within the transaction and the portion of each operation within the transaction is decided by the workload specification.

The current implementation is 1.
However, other projects such as DBx1000, cc-bench seems to be 2 so it might be better to change this implementation to 1 as well.