[dataimport] cleanup a bit nohook store, mostly protecting some attributes

[dataimport] add a filter function to not fail if some extentity has several values for an attribute of final relation the function will simply record a warning in the import log and keep a value randomly. Notice the function has to be explicitly inserted in the transformation stream.

[sources] synchronize source asynchronously when started from the UI and redirect to the forthcoming import log, for a better user experience (there is still a lot more to do there though). Closes #10468967