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.

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

javatests/google/registry/model/host/HostResourceTest.java

@@ -284,4 +284,10 @@ public class HostResourceTest extends EntityTestCase {
     assertThat(afterTransfer.getCurrentSponsorClientId()).isEqualTo("winner");
     assertThat(afterTransfer.getLastTransferTime()).isEqualTo(clock.nowUtc().plusDays(1));
   }
+
+  @Test
+  public void testToHydratedString_notCircular() {
+    // If there are circular references, this will overflow the stack.
+    hostResource.toHydratedString();
+  }
 }