a re-created cache signs with a new key; the fact was only created once, so
every hive then rejected the swarm's closures. the cache's grant gains update
on that one leaf, and a store that refuses it leaves the key as it was.
Closes#6181
the in-swarm cache stores its address, name and public key beside its tokens, create-only. swarm-controller copies the four cache facts to swarm/hives/<hive>/attic/* every five minutes, with read on the sources and write on the copies. part 1 of 3: nothing reads the copies until the hive reader and probe land.
