ref: Implement two-phase voting for DeleteRefs
The DeleteRefs RPC is voting on changes it's about to commit once after all references have been locked, but it doesn't do a second vote to confirm that deleting the refs has actually succeeded. Doing such two-phase voting will soon become mandatory though with the introduction of voting phases such that we can start to ignore errors if a transaction failed to reach quorum in "prepared" state.
Implement support for two-phase voting in DeleteRefs to prepare for this.
Changelog: changed
Part of #3709 (closed).