fsrvp: use fss server state storage back-end

fsrvp: use fss server state storage back-end

MS-FSRVP outlines the requirement for persistent server state storage,
retrieved on startup. Store fss server state following snapshot
creation, deletion and exposure.

Storage and retrieval errors are logged but otherwise ignored. In such
a case, manual snapshot cleanup may be required if the server is
restarted with snapshots in existence.

smap sc_share_name was not set until exposure time, meaning there was
nothing available as a unique identifier. The sc_share_name is now set
immediately on sc/smap creation.