Migrate xprofilemeta functions to the WP metadata API

The case of XProfile metadata is complex, because xprofile groups, fields, and
data all store their metadata in a single table, differentiated by object_type.
This complication requires additional filters on WP's core metadata queries.

See #4551

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7862 cdf35c40-ae34-48e0-9cc9-0c9da1808c22