Fix member expiration not always working

The Sidekiq job `RemoveExpiredMembersWorker` was failing to run in
production because it was hitting statement timeouts because it was
scanning all rows in order. On staging, where it used to scan 4 million
rows, adding an index brought this down to only a few hundred rows.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/67286

changelogs/unreleased/sh-add-index-members-expires.yml

+---
+title: Fix member expiration not always working
+merge_request: 32951
+author:
+type: performance

db/migrate/20190912061145_add_index_to_members_on_expires_at.rb

+# frozen_string_literal: true
+
+class AddIndexToMembersOnExpiresAt < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :members, :expires_at
+  end
+
+  def down
+    remove_concurrent_index :members, :expires_at
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_09_10_000130) do
+ActiveRecord::Schema.define(version: 2019_09_12_061145) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -1997,6 +1997,7 @@ ActiveRecord::Schema.define(version: 2019_09_10_000130) do
     t.boolean "ldap", default: false, null: false
     t.boolean "override", default: false, null: false
     t.index ["access_level"], name: "index_members_on_access_level"
+    t.index ["expires_at"], name: "index_members_on_expires_at"
     t.index ["invite_email"], name: "index_members_on_invite_email"
     t.index ["invite_token"], name: "index_members_on_invite_token", unique: true
     t.index ["requested_at"], name: "index_members_on_requested_at"