mod_log_json: Prefer native Lua table.pack over Prosody util.table one Prosody is removing support for Lua 5.1, which was the reason for util.table.pack to exist in the first place, since Lua 5.2+ provides table.pack. In prosody rev 5eaf77114fdb everything was switched over to use table.pack, opening the door for removing util.table.pack at some point. This change here is to prepare for that future eventuality.

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.