PR Visitor Meta Bug Fix + Visitor Meta Value Alias + Visitor Meta Database Upgrade

* Fix return value in utility function to return custom keys
* Fix typo in function call name
* Fix incorrect variable name in get custom values utility function

**New**
* Added feature to let devs alias visitor meta keys.  Use to make plugins compatible with each other and to address backwards compatibility issues as visitor meta evolves
* Added checks to prevent WPEC internal visitor meta keys from being aliased
* Added checks to prevent standard visitor meta keys from being aliases.  Standard visitor meta keys are to unique names used on checkout forms
* Added database upgrade routine that can be used in preparing future releases that will upgrade visitor meta keys to new names as required by devs.  Devs need to pass an array of old key / new key into the utility routine called from their upgrade routine.