Save a join to the Users table, when possible

It is unnecessary to join to CGM.MemberId to Users.id merely to limit on
Users.id; instead, limit directly on the MemberId.  This theoretically
lets through rows where the MemberId points to a non-existant User, but
such cases are the result of database corruption already.
Merge branch '4.2/rolelimit-skip-user-join'