Deprecate public access to some DifferenceEngine properties

Deprecate public access to some DifferenceEngine properties

These had no business being public in the first place and there is
no usage in Gerrit. In case something does use them, it will probably
be broken by the refactoring that's soon to come, so the deprecation
warning makes it easier to figure out where the problem lies.

Change-Id: I72ba5cfbf91f4af16028ba4f8619df6a7168a786