Prevent pallet version inconsistency

[ggwpez]

Currently we have the soft assumption that on_chain_storage_version() == current_storage_version() for all pallets.
There are indications that this does not hold for all pallets on Rococo. It would be nice to have a post-migration check which asserts this so that no migrations can be forgotten.

[bkchr]

Yeah, I have already started working on this. I need to finish it :P

[ggwpez]

I wrote a draft to check Rococo here master...oty-check-pallet-versions
Seems like there are issues with it. Does your code look much different @bkchr ?