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
mapper: Return error if map cannot be inserted to cache
mapper: Remove excessive logging on map node references
mapper: Fix number of object of uninitialized maps