nfs4: Fix gIdMapper resource leak

Mounting another nfs4 volume loses reference to the previous idmapper
service. Thanks Stephan for pointing this out.
nfs4: Make sure OpenDirCookie::fSnapshot is properly initialized
nfs4: Fix coding style

No functional change intended.
nfs4: Notify listeners when file grows
nfs4: Don't flush cache when closing read only share reservation

NFS4 specification requires that modified data has to be flushed before
sending CLOSE request. Our original implementation was stricter than that
by flushing all modified data at each file close. That caused problems
with e.g. Tracker, which likes to periodically open files for read only,
in a situation when there were still asynchronous writes pending
(e.g. large file copied).