Introduce meta_tables param for BP_Component

This utility method is used to set up configuration for $wpdb integration
and use of the WordPress metadata API

Also includes restructuring of the global_tables set in BP_Component.

See #4551

Props johnjamesjacoby

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7839 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Migrate groupmeta functions to the WP metadata API

See #4551

Props boonebgorges, johnjamesjacoby

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7840 cdf35c40-ae34-48e0-9cc9-0c9da1808c22