Customer meta hardening

PR for issue #845

* Added documentation describing customer profile implementation to customer.php*
* Added function to allow last active time to be set for users other than the current user.
* Avoid deleting users that have posts, comments or purchases associated with their user id.
* Fill in purchase log user id field with temporary profile id when purchase is completed.
* Adjusted the temporary profile delete processing and logic to eliminate unused temporary customer
profiles sooner.
* Added filters for hooking customer meta CRUD operations.paralleling what is available in WordPress It would be great to have these filters for each of our meta types.