Killfile for fields api

This represents the task for adding the support for killable fields in 7.x-2.x.
This will prevent permanent deletion of node fields and their data in the same manner as killable content.
This will allow for permanent deletion of fields and their node data.
This will allow for unkilling of fields and their node data.

Killing and unkilling will be an extra-careful case because multiple node types may each share the same field and only certain node types may need to be killed or unkilled.

Todo: It will make sense to provide a history for field and widget changes so even these can be undone.
This feature, however, branches out beyond the scope of killfile.
What needs to be done here is to write a separate module and then provide hooks so that killfile can depend on and add its own kill and unkill history.

Implementing this feature might solve this feature request: http://drupal.org/node/327089