cubicweb #1789761 Modification for large scale databe [open]

It should be possible to create "light" entities that do not require some meta data such as "owned_by", "created_by", "modification_date" and "creation_date". Such meta data take lots of space in memory and are not always used. Some redundancy in relations such as "is" and "is_instance_of" may perhaps be avoid.

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