cubicweb #4619277 migration: speed up [validation pending]

The current 3.18.0 migration is very slow and could use less commits.

prioritynormal
typebug
done in3.18.7
load0.100
load left0.000
closed by#b23043546f2c [migration/3.18] speed up the migration and be a little bit more informative
patch[migration/3.18] speed up the migration and be a little bit more informative [applied]