xrootd,nfs: Add support for space management

Xrootd and NFS doors fail to include the space manager in pool manager
requests, thus failing to support space management. Even though the
protocols don't have explicit support for space management, they may
still be used with the WriteToken tag or with implicit reservations.

If space manager is disabled, the message flow is unaffected. Even if
space manager is enabled, read requests or writes to unmanaged space
are simply passed through in space manager (which when space manager
is placed in the same domain as pool manager is fairly cheap).

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7049/
Request: 2.9
Request: 2.8
Request: 2.7