BUGFIX: Correct after_fork semantics

After fork SiteSettings was not getting a new process id,
causing site settings not to refresh properly in unicorn

This code also centralizes the logic

lib/site_setting_extension.rb

@@ -179,6 +179,10 @@ module SiteSettingExtension
     @@process_id ||= SecureRandom.uuid
   end
 
+  def after_fork
+    @@process_id = nil
+  end
+
   def remove_override!(name)
     provider.destroy(name)
     current[name] = defaults[name]