This patch enhances the sorcery API to allow for sorcery wizards to
determine if an object is stale. This includes the following:

* Sorcery objects now have a timestamp that is set on creation. Since
  sorcery objects are immutable, this can be used by sorcery wizards to
  determine if an object is stale.

* A new API call has been added, ast_sorcery_is_stale. This API call
  queries the wizards associated with the object, calling a new callback
  function 'is_stale'. Note that if a wizard does not support the new
  callback, objects are always assumed to not be stale.

* Unit tests have been added that cover the new API call.

Change-Id: Ica93c6a4e8a06c0376ea43e00cf702920b806064