Tweak `spawn_thread_pool`.
Tweak `spawn_thread_pool`.

This makes the two versions (parallel and non-parallel) more similar to
each other.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust