When user is a member of many groups (think of 5k or more), handling of `BE_REQ_INITGROUPS` (triggered by, for example, `id $user`) by generic SSSD LDAP backend (this ticket is specifically about RFC2307 setup, i.e. no nested groups) takes extremely long time (can be several mins). With membership large enough this can even exceed `SBUS_MESSAGE_TIMEOUT`, making 'sssd_nss' to abort client's request (so `id` returns nothing).

The purpose of this ticket is to track measurable performance improvements of described scenario.