cubicweb #2408814 add a new cwsource -> no immediate synchro can happen before a restart [validation pending]

Seen with an ldapuser source.
That's because on post creation, a new looping task is scheduled, but this is forbidden after startup.

Fix: allow task etc. to be created at any time.

prioritynormal
typebug
done in3.15.2
load0.200
load left0.000
closed by#cae198371548 [server/utils] add_looping_task can be used any time (closes #2408814)