This CL does a very similar cleanup to Cancellation as the diffbase
does to GracePeriod - see [] It simplifes logic where we
used to overload methods to accept eitherw OneTime or Recurring billing
events refs, despite storing them in separate fields in the entity
(since BillingEvent is not a polymorphic superclass, just a Java-only
one, you can't store them as refs to BillingEvent).
That overloading was ultimately only there as a convenience/hack from
when we added Recurring events and didn't want to go back and change
everything. It obfuscates what's really going on, requires extra
casting/loss of type-safety, and relies on indirect signals (e.g. the
Billing event reason being AUTO_RENEW) to guess what the right billing
event type is. That latter aspect will likely no longer work in a
monthly billing world, and was brittle anyways (as came up in the
context of removing the AUTO_RENEW reason - see []
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