Use recursive CTE when MPTT is not there
For 'child_of' domain, we have two strategy the MPTT and the recursive ORM loop.
Indeed when the database support the recursive CTE, we could use it as an alternative between both. Recursive CTE is not better than MPTT but still avoid many round-trip compared to the recursive ORM loop.
Normally, postgresql and sqlite supports recursive MPTT but it is still good to have a backend flag for external backend which does not support it.