user_ldap: Add support for gidNumber

This patch is based on the work of @dleeuw (https://github.com/dleeuw)
(See https://github.com/nextcloud/server/issues/2640#issuecomment-269615883 for more details).
The difference is user & group data will be written into cache to have
better performance, and functions splited from primaryGroupID series to
make them more readable.

Fixed https://github.com/nextcloud/server/issues/2640



Signed-off-by: Xuanwo <xuanwo@yunify.com>