The use of ProtobufCodec requires that value classes need to be
annotated with @Column, which won't be possible as soon as we have
removed gwtorm. Hence, provide a hand-written converter for
Change protobuf messages.
As protobuf Changes are currently used in indices
and we don't want to invalidate those, we have to ensure binary
compatibility. Prove that the new converter generates binary compatible
results via dedicated tests. Those tests will be removed after this
change.
Change-Id: Ie1dcd3dd28c628cb8c20d0f95147417e8b2fd260
921d9d67f9