[tools] Provide a faster build_user
The main trick is to use a cache of user entities.
To do so, a few tools are needed since the entities are not supposed to be
copied around between connexions.
Related to #4870347
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7ffca3b439105a0bcca1a6fdf83710be3083a4b1 pyramid_cubicweb-debian-version-0.1.0-1
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3d60138100d660e430e069c5d104ec927820d298 pyramid_cubicweb-debian-version-0.1.0-1
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1ae61c25299a6312acd09dc8d2f2c65e2e53f93b pyramid_cubicweb-debian-version-0.2.1-1