Adding tx option in setCommand

1] Why this feature is needed?
 - when someone wants redis to use a cache, They will do them following these sequence.
 1] write db
 2] write redis

 but after 1], because of context-switching or some other spending time, they can cause race condition. We assume there are 2 clients A, B
 
There is no problem.
 1) A - Write DB
 2) B - Write DB
 3) A - Write Redis
 4) B - Write Redis

But, 

 1) A - Write DB
 2) B - Write DB
 3) B - Write Redis
 4) A - Write Redis(Context Switching or other event)

so this time we should use global locking, but I think set tx option will help it

 1) A - Write DB (get that time)
 2) B - Write DB (get that time)
 3) B - Write Redis (with tx time : 1005000)
 4) A - Write Redis (with tx time : 1000000) <- so this operation will fail.

of course, it has some drawbacks.
1] using more memory when client use tx option
2] it can just pass "normal set command"
3] rdb format is changed, so previous version can't read next version.

Thank you.