Add unique constraints on domain_hosts (#911)

* Add unique constraints on domain_hosts

Add unique constraints on DomainHost (child of DomainBase) and
DomainHistoryHost (child of DomainHistory). DomainHost is non-entity
embedded object and Hibernate does not define indexes automatically.

This should improve read and write performance of the parent entities.

db/src/main/resources/sql/flyway/V83__add_indexes_on_domainhost.sql

@@ -0,0 +1,23 @@
+-- Copyright 2020 The Nomulus Authors. All Rights Reserved.
+--
+-- Licensed under the Apache License, Version 2.0 (the "License");
+-- you may not use this file except in compliance with the License.
+-- You may obtain a copy of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+
+alter table if exists "DomainHistoryHost"
+   add constraint UKt2e7ae3t8gcsxd13wjx2ka7ij unique (
+       domain_history_history_revision_id,
+       domain_history_domain_repo_id,
+       host_repo_id);
+
+alter table if exists "DomainHost"
+   add constraint UKat9erbh52e4lg3jw6ai9wkjj9 unique (
+       domain_repo_id, host_repo_id);