mod_log_ringbuffer: Detach event handlers on logging reload (thanks Menel)
Otherwise the global event handlers accumulate, one added each time
logging is reoladed, and each invocation of the signal or event triggers
one dump of each created ringbuffer.
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.