cubicweb #2719113 entities descriptor leads to memory fragmentation issue [validation pending]

descriptor on entity class used to access attribute and relation are lazily created when the first entity of a class is create. This leads to memory fragmentation issue.

priorityimportant
typebug
done in3.16.4
load left0.000
closed by#409ea1ed9832 [vreg] (re)build entity classes cache after registry initialization.
patch[vreg] (re)build entity classes cache after registry initialization. Closes #2719113 (potential memory fragmentation issue) [applied]