Cancel a merge

Refuses, in this order: a merge that isn't `rehearsed` (`merge_not_rehearsed`, or `merge_stale` for a stale one); a `version` that isn't the merge's (`merge_version_conflict`: a save of acknowledgements or the after-merge choice keeps a merge rehearsed, so send the version you last saw to apply what you saw); conflicts without a resolution (`merge_conflicts_open`); data-loss, security (BYPASSRLS, owner-level) or breaking (a role rename) steps without an acknowledgement (`merge_drops_unacknowledged`); a protected target without `confirm: true` (`merge_confirm_required`); another merge applying to the target, or failed half applied on it (`merge_busy`). The worker then runs the plan (`applying`, then `online` for the concurrent index and validation steps, then `finishing`). The request doesn't read the target: before the plan's transaction the worker compares the target with the schema the plan was computed from, and if it changed, ends the merge `stale` without applying the transaction.