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 logs events where more than a certain amount of CPU time was
spent.

``` lua
log_cpu_threshold = 0.01 -- in seconds, so this is 10 milliseconds
```

Uses the Lua
[`os.clock()`](http://www.lua.org/manual/5.2/manual.html#pdf-os.clock)
function to estimate CPU usage.