pool: Fix race leading to false positices in pool size health checks

The pool would register free space before actually deleting the file
from the file system. Thus for a brief moment, the pool would appear
to have more free space that is available on disk.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7367/
(cherry picked from commit dbd6b7043bca392f4ef5487b5889dadd828fec31)
Merge pull request #951 from gbehrmann/fix/2.6/rb7367

pool: Fix race leading to false positices in pool size health checks