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).
At its start the keeper reads the log into runs once, prunes old records and carries unfinished runs on;
after every released lock it carries them on again; it takes each ask at <node>.* and prunes records daily.
The root helper's client is a constructor (newRootActs) the helper replaces; until then every act is
refused, named.
The keeper reads pacman's database into installed.<node> and writes it only when it differs from the key,
says each change as package.changed, and keeps the last value when the list is over the bus's cap, marking
the node not current until it fits again; the health names the size. The loop runs over injected states,
events, clock, watch and machine, and leaves hooks (start, after a transaction, state watches, wakes) for the
upgrade parts of ADR 0508.
jschoubben