Improve the performance of CAS

I'm working on some performance improvements of the lightweight transitions (compare and set).
 
As you know, current CAS requires 4 round trips to finish, which is not efficient, especially in cross DC case.
1) Prepare
2) Quorum read current value
3) Propose new value
4) Commit
 
I'm proposing the following improvements to reduce it to 2 round trips, which is:
1) Combine prepare and quorum read together, use only one round trip to decide the ballot and also piggyback the current value in response.
2) Propose new value, and then send out the commit request asynchronously, so client will not wait for the ack of the commit. In case of commit failures, we should still have chance to retry/repair it through hints or following read/cas events.
 
After the improvement, we should be able to finish the CAS operation using 2 rounds trips. There can be following improvements as well, and this can be a start point.