pacman_rollback plans from the bus: each package now -> record from the node's cache, else the archive's
exact version, else the archive's repository on the record's date; added packages listed and removed only
when asked; foreign packages untouched. apply is an ask the node's keeper runs as a rollback run through the
root helper's new Rollback act (RootActs, upgrades.go), with rollback.started, rollback.finished and the
after-upgrade check (TakeAsk's rollback case, transaction.go).
