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
mapv_t type does not propose to change uint32_t version defined in struct map (maybe my mistake defining it after typedef). I believe you can freely use the proposed enumerator to declare the different map file versions and make code visual handy and easier to scan.