Rotate the stream key

Any of the fields; the others stay. The checks are create's: `viewer_cap` at least 1 and at most the tenant's (409 `tenant_viewer_cap`), `viewer_cap` plus `queue_max` at most 20000, and seats and places in line across every stream at most 20000 (409 `too_many_places`). Setting `viewer_cap` alone keeps `queue_max`. Lowering the cap below the viewers seated keeps them in (their tokens keep refreshing) and seats nobody new until they are under it; an offer the new cap cannot keep is withdrawn and its page stays first in line. A lowered `queue_max` keeps the pages in line and takes nobody new until under it. Changing `admission` changes only how new viewers are admitted; held seats keep working. Namespace, mode, ingest and `max_kbps` cannot be changed (create a new stream).