Index the allocation token's redemption history entry field

We need this to efficiently query which tokens have and have not been redeemed.

I'm also marking it as nullable for consistency with the domain name field (it's
already null for unredeemed tokens).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214366534

javatests/google/registry/model/domain/token/AllocationTokenTest.java

@@ -48,10 +48,12 @@ public class AllocationTokenTest extends EntityTestCase {
         persistResource(
             new AllocationToken.Builder()
                 .setToken("abc123")
+                .setRedemptionHistoryEntry(Key.create(HistoryEntry.class, 1L))
                 .setDomainName("blahdomain.fake")
                 .setCreationTimeForTest(DateTime.parse("2010-11-12T05:00:00Z"))
                 .build()),
         "token",
+        "redemptionHistoryEntry",
         "domainName");
   }