og_user_roles_user_manage() does not scale

Currently, og_user_roles_user_manage() works by doing a db_query('SELECT uid FROM {users}') and user_load-ing every entry.

For medium and large sites, this means a totally unrealistic load both on the DB (one full scan table + n queries) and on the PHP (user_load-ing all user accounts).

The purpose of this code is to obtain a list of the accounts in groups managed by the current user, and only them, filtering achieved by an array_intersect() after building that monster list.

This should be done in a cheaper manner, and seems possible with just one query over the {og_uid} table.