spacemanager: Set file to TRANSFERRING on accept file request message

By changing the file entry to TRANSFERRING on the accept
file request rather than response we save:

- A database lookup
- A database update
- A message queueing phase

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6547/