pool: Fix concurrency settings for script nearline storage

Adjusting the thread pool size did not have an immediate effect. This
patch resolves the problem.

Target: trunk
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7181/