R1:237f94b830a4

Fix flaky subscribers policy rule unit test

Summary:
I'm about 90% sure this fixes the intermittent test failure on `testObjectSubscribersPolicyRule()` or whatever.

We use `spl_object_hash()` to identify objects when passing hints about policy changes to policy rules. This is hacky, and I think it's the source of the unit test issue.

Specifically, `spl_object_hash()` is approximately just returning the memory address of the object, and two objects can occasionally use the same memory address (one gets…