spacemanager: Use FsPath to represent file system paths

I find using FsPath rather than just String to represent paths
in our name space often makes the code cleaner. This patch
update space manager to use FsPath in this way.

The path removes a number of unused fields from our messages and
thus simplifies the logic in space manager.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6465/