mod_rest: Expand mapping of XEP-0045 join stanza The previous 'join' mapping was apparently lost in translation when swithing to datamapper, so might as well map some properties allowing history control. Usually you probably want either zero history or history since the last known time of being joined. Maybe that the former should be the default?

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.