Add an index on Host.host_name column (#1510)

* Add an index on Host.host_name column

This field is queried during host creation and needs an index to speed
up the query.

Since Hibernate does not explicitly refer to indexes, we can change the
code and schema in one PR.

db/src/main/resources/sql/flyway/V105__add_index_on_host_name_in_host_table.sql

@@ -0,0 +1,15 @@
+-- Copyright 2022 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.
+
+CREATE INDEX IDXkpkh68n6dy5v51047yr6b0e9l ON "Host" (host_name);