[NoWait, WaitDie] Need downgrade of rw-lock

[rotaki]

https://github.com/wattlebirdaz/tpcc-runner/blob/fdfc8f0759705e7a95989557fe6837842af44871/protocols/nowait/include/nowait.hpp#L68-L135

Unlocking the next-key in insert operation causes unlocking of read-lock if a read to the next-key precedes the insert. To fix this, a method to downgrade locks from W->R is needed.