Add optimistic lock to avoid race conditions when handling votes (#10196)

* Add optimistic lock to avoid race conditions when handling votes

* Force-reload polls when getting `ActiveRecord::StaleObjectError`

db/migrate/20190306145741_add_lock_version_to_polls.rb

@@ -0,0 +1,6 @@
+class AddLockVersionToPolls < ActiveRecord::Migration[5.2]
+  def change
+    add_column :polls, :lock_version, :integer, null: false, default: 0
+  end
+end
+