fsrvp: add server state storage back-end

fsrvp: add server state storage back-end

MS-FSRVP specifies:
  the server MUST persist all state information into an implementation-
  specific configuration store.

This change adds a fss_srv TDB database to preserve FSRVP server state,
with the following keys used to track shadow copy state and hierarchy:
- sc_set/<shadow copy set GUID>
  A shadow copy set tracks a collection of zero or more shadow copies,
  as initiated by a StartShadowCopySet FSRVP client request.
- sc_set/<shadow copy set GUID>/sc/<shadow copy GUID>
  A shadow copy defines information about a snapshot base volume, the
  snapshot path, and a collection of share maps. It is initiated by an
  AddToShadowCopySet client request.
- sc_set/<shadow copy set GUID>/sc/<shadow copy GUID>/smap/<smap GUID>
  A share map tracks new shares that are created to expose shadow
  copies.

All structures are marshalled into on-disk format using the previously
added fsrvp_state IDL library.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>