account_deletions get stuck while selecting participations

Following query takes over 20 minutes and fills up the database:
SELECT * FROM participations WHERE author_id = XXXXX

On servers with a huge participations-table this is a serious
performance issue which is easily fixed by the author_id index

Signed-off-by: Lukas Matt <lukas@zauberstuhl.de>

db/migrate/20160327103605_add_author_id_index_to_participations.rb

+class AddAuthorIdIndexToParticipations < ActiveRecord::Migration
+  def change
+    add_index :participations, :author_id, :using => :btree
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160307142216) do
+ActiveRecord::Schema.define(version: 20160327103605) do
 
   create_table "account_deletions", force: :cascade do |t|
     t.string   "diaspora_handle", limit: 255
@@ -320,6 +320,7 @@ ActiveRecord::Schema.define(version: 20160307142216) do
     t.integer  "count",            limit: 4,     default: 1, null: false
   end
 
+  add_index "participations", ["author_id"], name: "index_participations_on_author_id", using: :btree
   add_index "participations", ["guid"], name: "index_participations_on_guid", length: {"guid"=>191}, using: :btree
   add_index "participations", ["target_id", "target_type", "author_id"], name: "index_participations_on_target_id_and_target_type_and_author_id", using: :btree