perf(session): speed up elgg_is_admin_user()

This function currently makes one query and generally hits the query cache
several more times. This generally eliminates the query and avoids the overhead
of the DB service. If a query must be done we use a minimal result set.
Merge pull request #6947 from mrclay/is_admin_user

perf(session): speed up elgg_is_admin_user()