This introduces two caches, svc_export_cache (nfsd.exports) and
svc_expkey_cache (nfsd.fh).
nfsd.exports maps client+directory  ->  export options.
nfsd.fh maps client + filehandle-fragment -> directory.

A major part of this change is that export entries are now reference counted,
so we have to be careful  to keep those counts correct.