Update Exchange touch on Enterprise to use touch_later

This can improve issues with deadlocks where multiple touch calls are triggered in a single update, for example where an exchange touches it's parent enterprise on save, and we do this:

oc_with_lots_of_exchanges.exchanges.each{|ex| ex.clone! }

app/models/exchange.rb

@@ -12,7 +12,7 @@ class Exchange < ActiveRecord::Base
 
   belongs_to :order_cycle
   belongs_to :sender, class_name: 'Enterprise'
-  belongs_to :receiver, class_name: 'Enterprise', touch: true
+  belongs_to :receiver, class_name: 'Enterprise'
 
   has_many :exchange_variants, dependent: :destroy
   has_many :variants, through: :exchange_variants
@@ -23,6 +23,8 @@ class Exchange < ActiveRecord::Base
   validates :order_cycle, :sender, :receiver, presence: true
   validates :sender_id, uniqueness: { scope: [:order_cycle_id, :receiver_id, :incoming] }
 
+  after_save :touch_receiver
+
   accepts_nested_attributes_for :variants
 
   scope :in_order_cycle, lambda { |order_cycle| where(order_cycle_id: order_cycle) }
@@ -95,4 +97,10 @@ class Exchange < ActiveRecord::Base
   def participant
     incoming? ? sender : receiver
   end
+
+  def touch_receiver
+    return unless receiver&.persisted?
+
+    receiver.touch_later
+  end
 end