Sort the host_ids for each hostgroup

Sort the host_ids for each hostgroup in the '_id_by_hostgroup_name_heap'. This allowed preselected hosts to be sorted by hostname when using the hints in livestatus_query_metainfo.

This is to fix https://github.com/shinken-monitoring/mod-livestatus/issues/22

From the look of the code this issue may also affect _id_by_servicegroup_name_heap and _id_by_hostgroup_name_heap ?