[repo, undo] Finish repository-side implementation of the undo feature (closes #893940)
1) Changes API for undo_transaction in order to raise an exception:
the previous API returned a sequence of error, the user of the API
needed to be careful to check explicitly. We change this for it to
raise an UndoException with the list of errors as an attribute.
2) The server-side support for undoing update actions is finished
3) The configuration for `undo-support` is now a simple boolean
Beware of the costs, both in DB space and in number of requests
on the RDBMS backend, once this feture is activated.
4) Adds a txuuid param/attribute to the existing NoSuchTransaction
exception.