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
a) This setting affects the default test config, where the filed directory is created under /tmp. To avoid having tests fail if /tmp is mounted as tmpfs, we disable directIO by default.

b) After converting from v0 to v2, there is no Pithos involvement anymore. The whole issue was raised, when Pithos was creating map files on its own, without going through the mapper layer. With our integration plan, Pithos will just be another endpoint for Archipelago, and will go through it to access the data. Archipelago itself, works well with the directIO setting in filed.