Add capabilities to include import by reference between dataspace

Currently when objects need to be available in two different dataspace, objects (XML and arrays) needs to be copied, duplicating the storage. The alternative is to introduce the notion of import by reference, where the object will be known from the destination dataspace but most of its storage will remains in the original dataspace.

• In order to avoid referential integrity issues, the original dataspace must be in read-only mode, so locking/unlocking dataspace must be implemented, and the dataspace must remain in read-only mode as long as it is referenced
• The access control should be checked on the accessed dataspace, wherever the resources are physically stored there or referenced. A new protocol needs to be implemented to allow these functionalities (dataspace import by reference and locking) and new command needs to be implemented for the client.