Customer meta hardening

Added documentation describing customer profile implentation 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 thier 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 paralleling what is available in WordPress for hooking customer meta CRUD operations.
It would be great to have these filters for ewach of our meta types.