Allow to store AP entities coming from other instances

The first implementation of the storage only allow to store AP entities that concern the current instance. However this solution is not reliable on the long term for some reasons: some objects might not be available anymore on an instance, some instances might be disconnected, it involves to always query back the info on the network, it doesn't allow to perform queries concerning objects from other instances (AP user picker), etc.

All in all, it seems more interesting to allow to store locally the information concerning those entities when we need to.