Rewrites an event's subject as a stream's owning client knows the person:
an iss_sub user (or a complex subject's user and device) becomes that
client's pairwise or ephemeral value. A public client, and a stream no
client owns, get the subject unchanged.
Rewrites an event's subject as a stream's owning client knows the person: an
iss_subuser (or a complex subject'suseranddevice) becomes that client's pairwise or ephemeral value. A public client, and a stream no client owns, get the subject unchanged.