cubicweb #1625325 consider inlining created_by [open]

created_by has the required cardinality for being inlined. Test to see if speed is gained from doing this:

  • I expect setting the relation will be faster, especially in massive entity creation context
  • I expect lookup of the creator of an entity to be faster
  • I expect finding entities of different type created by a user to be slower, but I think doing this kind of lookup should be rare.
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