mod_pubsub_serverinfo: Warm-up opt-in cache By warming up the cache that contains the opt-in data, the first publication has a better chance of including domain names for remote domains that opt-in. Without this change, those domains are named only after the _second_ publication, which can take a while. New users are likely thrown off by 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.