Add create timestamp as field on AllocationToken entity

This definitely seems like a useful thing to have around.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181401799

java/google/registry/model/EppResource.java

@@ -202,7 +202,8 @@ public abstract class EppResource extends BackupGroupRoot implements Buildable {
      * normal EPP flows.
      */
     public B setCreationTime(DateTime creationTime) {
-      checkState(getInstance().creationTime.timestamp == null,
+      checkState(
+          getInstance().creationTime.getTimestamp() == null,
           "creationTime can only be set once for EppResource.");
       getInstance().creationTime = CreateAutoTimestamp.create(creationTime);
       return thisCastToDerived();