Visitor Profile and Meta API implementation that does not use WordPress Users

No user records, no user meta, fully backwards compatible.

Also includes:
* Staged cleanup of expired visitors, respecting memory and processing time.
* New visitors expire quickly, longer term visitors expire later.
* Profiles are not automatically deleted if profile becomes connected to a wordpress user with posts, comments or purchases,
* AJAX Meta API, get and set current user meta from javascript/jQuery.
* Upgrade routine to move WordPRess user based profiles to new infrastructure without loss of cart or other information

Status:
* Unit testing done, ready for integration testing ( ps. I hid 5 bugs in the code to be sure you tested it well, see if you can find them! )
* Needs upgrade testing
* Needs Install testing
* One more fix to prevent multiple profile creation when WPEC is using page caching
* Need to make the PR automatically mergeable