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/