This fixes a regression introduced in #32518. In that PR, we removed the
slowdown logic that would throttle lookups when the table runs empty.
Said logic was originally added in #20389.
Usually it's fine, but there exist pathological cases, such as hive
tests, where the node can only discover one other node, so it can only
ever query that node and won't get any results. In cases like these, we
need to throttle the creation of lookups to avoid crazy CPU usage.
7c107c2691