Change @DoNotHydrate to work on fields, not types.

There was a circular reference when hydrating a domain with a
subordinate host, since the host references the domain. To fix
this, I redid @DoNotHydrate to be the way it should have been,
rather than the hack I had originally submitted. I also beefed
up the unit tests of the epp resource types to check for cycles.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135792416

java/google/registry/model/billing/BillingEvent.java

@@ -87,6 +87,7 @@ public abstract class BillingEvent extends ImmutableObject
   long id;
 
   @Parent
+  @DoNotHydrate
   Key<HistoryEntry> parent;
 
   /** The registrar to bill. */