check last_write_at field when applying rate limits to channels

db/migrate/20150311201046_add_last_write_at_to_channels.rb

@@ -0,0 +1,6 @@
+class AddLastWriteAtToChannels < ActiveRecord::Migration
+  def change
+    add_column :channels, :last_write_at, :datetime
+  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: 20141024161555) do
+ActiveRecord::Schema.define(version: 20150311201046) do
 
   create_table "active_admin_comments", force: true do |t|
     t.string   "namespace"
@@ -105,6 +105,7 @@ ActiveRecord::Schema.define(version: 20141024161555) do
     t.string   "user_agent"
     t.string   "realtime_io_serial_number", limit: 36
     t.text     "metadata"
+    t.datetime "last_write_at"
   end
 
   add_index "channels", ["latitude", "longitude"], name: "index_channels_on_latitude_and_longitude", using: :btree