Continuation of 6db0aca5

After 6db0aca5 we acquire assoc_mgr_lock for QoS outside of the loop,
in assoc_mgr_get_admin_level we need to acquire assoc_mgr_lock for user.
This is technically OK, but we want to avoid nested use of
assoc_mgr_lock, which is checked by _store_locks assertion.

validate_operator is often called in loops while
assoc_mgr_get_admin_level is a heavy operation. This commit changes the
functions to lighter versions geting slurmdb_user_rec_t instead of uid.

Bug 11257