pnfsmanager: Fix expected size extraction and tag caching

The expected file size tag is not extracted correctly when creating new
files. The tag caching also wasn't as effective as intended: To fix
this I had to add caching of the parent tag.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.9
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6975/