mod_http_admin_api: restructure group-related info in API - Return the members of the group right in the get_group_by_id call. This is an O(1) of extra work. - Remove the groups attribute from get_user_by_name as that is O(n) of work and rarely immediately needed. The replacement for the group membership information in the user is for now to use the group API and iterate; future work may fix that.

This module helps reduces power usage of inactive mobile clients while
another client is actively used. E.g. in the case of chatting on a
desktop computer, instantly pushing messages to a phone in someones
pocket is not the best use of radio time.

Works with [mod_csi_simple][doc:modules:mod_csi_simple] which is
included with Prosody since 0.11