Improve performance of Principal.member_of scope

The 'join' turns to be slow for large numbers of members (>20k). Using a
sub-query like in Principal.not_member_of is more efficient and performance
is the same for smaller values.


git-svn-id: http://svn.redmine.org/redmine/trunk@13139 e93f8b46-1217-0410-a6f0-8f06a7374b81