Title: #332: sydb: index improvements
I think the patches are great and we've seen already reports by users who run the
patches and reported that performance improved substantially.
There are some objects that are not user or group but still use OBJECTCLASS and not
CATEGORY. At least `sysdb_save_autofsmap` still uses `SYSDB_OBJECTCLASS`, then
`sysdb_svc_add` (or rather the sysdb_services module), the sysdb_ssh module,
sysdb_netgroups module and the sudo module.
See the full comment at https://github.com/SSSD/sssd/pull/332#issuecomment-323966202