cubicweb #2204047 Start splitting background tasks handling from the Repo object [validation pending]

This is a step toward a light weigth repo and decidated server object.

prioritynormal
typeenhancement
done in3.15.0
load1.000
load left0.000
closed by#b5d5a5630649 [repository] split repo initialization from starting looping task (closes #2204047)