mod_http_muc_log: Correctly handle changed or retracted reactions Since per XEP-0444 each reaction should overwrite all previous reactions on a particular message from a particular occupant. Previously repeated reactions would be counted again and retractions were not handled.

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.