No 429 for a busy key right after an update of Runtime

A key whose account runs many sandboxes may send up to 8 times the ordinary rate. Just after Runtime updated its servers, its first requests met the ordinary rate instead, for the moment it took to recognise the key, so a burst sent then was answered 429 rate_limited. Those requests now wait that moment, at most two seconds, and are served at the key's own rate.