When the `/keys/query` API is hit on client_reader worker Synapse may
decide that it needs to resync some remote deivces. Usually this happens
on master, and then gets cached. However, that fails on workers and so
it falls back to fetching devices from remotes directly, which may in
turn fail if the remote is down.
This allows registration to be handled by a worker, though the actual
write to the database still happens on master.
Note: due to the in-memory session map all registration requests must be
handled by the same worker.
