pool: Fix several minor issues in nearline storage subsystem

The pool now cancels flush and stage requests when the file is deleted.

Nearline storage configuration is now applied atomically rather than
one attribute at a time.

Target: trunk
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7172/
(cherry picked from commit c272c484ebfd4a95ab81b6e88908fed3ae771f78)
Merge pull request #710 from gbehrmann/fix/2.9/rb7172

pool: Fix several minor issues in nearline storage subsystem