Fix timing issues on convertion migration award emoji

This commit does two things:
1. It adds logic which prevents timing issues when running the
migration. During the migration, notes can be created which _should_
be award emoji and thus migrated. To prevent these timing issues, a
lock is obtained on the table (MySQL) or on Transaction level (PG).
2. There was no down migration before as you'd probably lose some data.
Data effected is all awards on notes. These could be migrated back, as
the noteable type would just be Note, though this would litter the DB
with data which should not be there. This down migration does not yet
delete the table.

db/migrate/20160416182152_convert_award_note_to_emoji_award.rb

 # rubocop:disable all
 class ConvertAwardNoteToEmojiAward < ActiveRecord::Migration
-  def change
+  disable_ddl_transaction!
+
   def up
-      execute "INSERT INTO award_emoji (awardable_type, awardable_id, user_id, name, created_at, updated_at) (SELECT noteable_type, noteable_id, author_id, note, created_at, updated_at FROM notes WHERE is_award = true)"
+    if Gitlab::Database.postgresql?
+      migrate_postgresql
+    else
+      migrate_mysql
+    end
+  end
 
-      execute "DELETE FROM notes WHERE is_award = true"
+  def down
+    add_column :notes, :is_award, :boolean
+
+    # This migration does NOT move the awards on notes, if the table is dropped in another migration, these notes will be lost.
+    execute "INSERT INTO notes (noteable_type, noteable_id, author_id, note, created_at, updated_at, is_award) (SELECT awardable_type, awardable_id, user_id, name, created_at, updated_at, TRUE FROM award_emoji)"
+  end
+
+  def migrate_postgresql
+    connection.transaction do
+      execute 'LOCK notes IN EXCLUSIVE'
+      migrate_notes
+    end
+  end
+
+  def migrate_mysql
+    execute 'LOCK TABLES notes WRITE'
+    migrate_notes
+  ensure
+    execute 'UNLOCK TABLES'
   end
+
+  def migrate_notes
+    execute "INSERT INTO award_emoji (awardable_type, awardable_id, user_id, name, created_at, updated_at) (SELECT noteable_type, noteable_id, author_id, note, created_at, updated_at FROM notes WHERE is_award = true)"
+    execute "DELETE FROM notes WHERE is_award = true"
+    remove_column :notes, :is_award, :boolean
   end
 end

db/migrate/20160416190505_remove_note_is_award.rb

-# rubocop:disable all
-class RemoveNoteIsAward < ActiveRecord::Migration
-  def change
-    remove_column :notes, :is_award, :boolean
-  end
-end