Holds placed by the rate limit gate are written to a new upstream_holds
table, and each instance reads that table at most every two seconds and
refuses requests to a host any instance holds. Before, every replica had
to get its own 429 before it stopped sending.
Instances waiting on a fetch lease reread the holds when the fetch fails,
so when the holder was rate limited they answer 503 with its Retry-After
instead of each sending one more request to the limited host.
