model change related to scheduler

policy table has version column that gets updated whenever its
respective event is added to event table. scheduler ensures that version
matches before executing event. This way, we dont bother deleting event
whenever a policy is deleted or updated. When a policy is updated, a new
event with new version is added. The old event when picked up will not
get executed since its respective policy (with that version) is not
there. Added version=None to get_policy that checks the version before
returning.

Refactored test_cass_models into seperate class for testing add_policy
and update_policy