Merge indexes for reblog on statuses table (#5831)

We added an index for `[account_id, reblog_of_id]`, but we already have a similar index for `reblog_of_id`. Those index will be bigger according to statuses count. For example, `reblog_of_id` index uses 800MB for 10GB statuses table.

So this patch swaps indexed columns like `[reblog_of_id, account_id]`, then it will covers both usage with single index.

Since those index creation may take a while, I've also disabled previous index creation.

db/migrate/20171125190735_remove_old_reblog_index_on_statuses.rb

@@ -0,0 +1,14 @@
+class RemoveOldReblogIndexOnStatuses < ActiveRecord::Migration[5.1]
+  disable_ddl_transaction!
+
+  def up
+    # This index may not exists (see migration 20171122120436)
+    remove_index :statuses, [:account_id, :reblog_of_id] if index_exists?(:statuses, [:account_id, :reblog_of_id])
+
+    remove_index :statuses, :reblog_of_id
+  end
+
+  def down
+    add_index :statuses, :reblog_of_id, algorithm: :concurrently
+  end
+end