Race condition when adding composite and immediate membership

There's a race condition that can allow you to add a composite and immediate membership to a group, which is not supposed to happen. The chance of this is higher pre-2.1.0 if a composite takes a long time to get created (because it would result in a lot of composite memberships). The fix for 2.1.0 is to make sure there isn't a conflict after the database updates are made but before the commit.