Drop foreign key constraints on PollMessages (#875)

* Drop foreign key constraints on PollMessages

PollMessages are deleted after being acked by recipients.
Other tables cannot have foreign key constraints on them.

db/src/main/resources/sql/flyway/V79__drop_foreign_keys_on_pollmessage.sql

@@ -0,0 +1,21 @@
+-- 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 ONLY public."Contact"
+    DROP CONSTRAINT fk_contact_transfer_gaining_poll_message_id,
+    DROP CONSTRAINT fk_contact_transfer_losing_poll_message_id;
+
+ALTER TABLE ONLY public."Domain"
+    DROP CONSTRAINT fk_domain_autorenew_poll_message_id,
+    DROP CONSTRAINT fk_domain_deletion_poll_message_id;