ensureDerived and updateRelations call each other too often

[sveneberth]

We have a loop in the ensureDerived and updateRelations.

Add a new derive size to your FileBone (image) --> Save the skeleton with it --> ensureDerived task is spawn --> ensureDerived generate new derive blob and calls updateRelations --> updateRelations saves the skeleton with the new derive meta --> FileBone postHandler calls ensureDerived again --> ensureDerived detects there is nothing to do.

The second ensureDerived call can be saved if the postHandler would know it's called from a updateRelations task caused by a ensureDerived change.