mod_http_oauth2: Return Authentication Time per OpenID Core Section 2 Mandatory To Implement, either MUST include or OPTIONAL depending on things we don't look at, so might as well include it all the time. Since we do not persist authentication state with cookies or such, the authentication time will always be some point between the user being sent to the authorization endpoint and the time they are sent back to the client application.

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.