cubicweb-container #2557494 replace <container_rtype> with container_root, wipe container_etype [open]

This will simplify the code quite a bit and we get back one full slot for each container entity. The migration will be a bit tough for some existing apps, and the API will change.

rationale: the amount of flexibility given by the current organisation is not really worth the runtime & db size price.

done in3.0.0
load left2.000
closed by<not specified>