pool: Prevent interruption of replica deletion during HSM flush

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7335/
(cherry picked from commit be918388ba1fb1899b2e2409ed8d9a4361cb3d07)
Merge pull request #866 from gbehrmann/fix/2.9/rb7335

pool: Prevent interruption of replica deletion during HSM flush