nfs: Prevent NFS door from starting if schema is not up to date

Although we do not want NFS doors to update the Chimera schema
(PnfsManager does that), we should still check whether the schema
is up to date.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6505/