Add basic AllocationToken validation/redemption for domain creates

The next step is to add them for domain checks as well (which is simpler
because it doesn't involve validation).

This requires the addition of a TrimWhitespaceAdapter for XML JAXB objects,
which will prove useful for other @XmlValue attributes in the future.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181526726

java/google/registry/model/domain/AllocationToken.java

@@ -73,6 +73,7 @@ public class AllocationToken extends BackupGroupRoot implements Buildable {
     }
 
     public Builder setToken(String token) {
+      checkState(getInstance().token == null, "token can only be set once");
       checkArgumentNotNull(token, "token must not be null");
       checkArgument(!token.isEmpty(), "token must not be blank");
       getInstance().token = token;