rust-update: make `update_from_null` respect `worker.numcpu` config option...
rust-update: make `update_from_null` respect `worker.numcpu` config option
This was overlooked in the original series.
This is important for tests (because we run many at once), and for the
occasional end user that wants to keep their CPU usage in check.
A future series should clean up this `worker` parameter tunelling business by
rewriting the config in Rust, but doing so on stable would be a very bad
idea.
