mod_muc_markers: Add option for @id rewriting, default off (may break some clients) XEP-0333 was updated to clarify that stanza-id should be used instead of the 'id' attribute when in a MUC. Some clients still use the id attribute, which is why we were rewriting it. Rewriting is bad because mod_muc advertises stable_id, indicating that Prosody does *not* rewrite ids. Recent versions of Conversations actually depend on this being true. All clients should migrate to using stanza-id for markers. See XEP-0333.

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.