mapper: Fix delete map data state

Add delete map data state and use it when appropriate
mapper: Add delete map handling function
mapper: Add seperate key field to maps

Add seperate key filed to maps, to be used for cache indexing purposes.
mapper: Use the new key field when caching map
mapper: Add change volume target functionality for maps
mapper: Use delete map function when destroying volume

Use the generic delete map function when destroying a volume. Also use the
approriate map state.
mapper: Add DESTROYING map state
mapper: Remove excessive logging on map node references
mapper: Introduce seperate get_ready_map function

When loading a resource, use a new seperate get_ready_map function to simplify
the process.
mapper: Return error if map cannot be inserted to cache
mapper: Fix number of object of uninitialized maps
mapper: Refactor rename function

Add seperate rename function and make rename code path use it
mapper: Use fallback map with the 'archip_' prefix

If loading a map with its name fails, try falling back to the volume prefixed by
'archip_'. Then, if this succeeds and we have exclusive access to the map, to a
permanent rename to get rid of the archip prefix.

The above is needed to fasciliate live volume migrations to the new naming
scheme.
mapper: Use purge when renaming 'archip_' maps
vlmc: Disallow creating volumes starting with 'archip_'

Since the 'archip_' prefix was used to differentiate archipelago from pithos
objects, keep it reserved and do not allow creating resources with this prefix.
vlmc: Drop the old prefix when manually handling locks