schema: ban replicaset UUID update

It wasn't allowed to drop it, but was allowed to update. The patch
bans it. Firstly, it was not supposed to work. Secondly, a future
patch will introduce a new tuple in _schema, which would store
replicaset UUID too. It won't allow UUID update.

Would be strange, if the update would be let through one _schema
tuple and wouldn't work via another.

Needed for #5029

NO_DOC=bugfix