cubicweb #2517096 one startup hook for each started looping task [done]

so one can easily replace one to remove/modify a task without bothering with the others

prioritynormal
typeenhancement
done in3.15.6
load0.100
load left0.000
closed by#bde0501e0251 [startup hook/looping tasks] separated hook for each looping task to ease modification from cubes. Closes #2517096