objects.base: reset changes after getting objects from database

objects.base: reset changes after getting objects from database

Now all objects are comparable.

We need to reset changes, otherwise an object that is constructed and
.create()d is different from the one that is .get_by_id()d from database
(for primitive serialization contains list of changed fields for
versioned objects).

Added initial sql test case for objects (just create-fetch for policy
for now, but can be easily extended to other types).

Change-Id: I012b5fe4e95f166f66da91274734d7184c224dfd