poolmanager: Shut down request container thread pool during shutdown

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7376/
(cherry picked from commit e9595f5da4a0adec702222c5404a5b29a97ac2a0)