ensure atomic updates for ListConfig sliced assignment (#950)

[pixelb]

• omegaconf/listconfig.py (setitem): Operate on a copy so any changes due to validation errors etc. are discarded. Fixes issue #950

[pixelb]

This works but is not very efficient. It would be good to have a replace() method I think, that would avoid all the assignment checks etc. Also ideally we would shallow copy the work list in this case.

[pixelb]

I'll push this now, and we can optimize later