Optimize retrieving display name when searching for users in a group

Optimize retrieving display name when searching for users in a group

This is recurrent scenario that we are searching for users and then for
each users we fetch the displayName. This is inefficient, so instead try
to do one query to fetch everything (e.g. Database backend) or use the
already existing DisplayNameCache helper.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>