kvcoord: transform replicated, locking Get requests

A replicated locking Get request is a write that we would like to
defer until commit time. We do this by transforming it to an
unreplicated locking Get. At flush time, if no other write has been
performed on that key, we send a replicated locking Get to ensure we
have exclusion on the key all the way until the commit timestamp.

Epic: none
Release note: None

Author: stevendanna