Feature pithoschanges

Includes the necessary changes for better integration with Pithos

Noticeable changes:

* Add signature for v2 maps
* Add v0 map hint
* Support map creation from a given object list
* Remove "archip_" prefix from maps and blocks created by archipelago
* Add rename functionality
* Immediately delete writeable objects when removing a map
* Add readonly option when mapping a volume
Indeed, the __cpu_to_type is a good way to go (and a more general one if we wish to target clusters with mixed architectures), but i believe it is out of scope of the current patch set. If you agree, we can proceed with the submitted version, and leave the proposed code refactoring (the proposed changes affect other parts of the mapper too) for another patch.