mod_firewall: Update user marks to store instantly via map store The original approach was to keep marks in memory only, and persist them at shutdown. That saves I/O, at the cost of potentially losing marks on an unclean shutdown. This change persists marks instantly, which may have some performance overhead but should be more "correct". It also splits the marking/unmarking into an event which may be watched or even fired by other modules.

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.