mod_http_oauth2: Implement refresh token rotation Makes refresh tokens one-time-use, handing out a new refresh token with each access token. Thus if a refresh token is stolen and used by an attacker, the next time the legitimate client tries to use the previous refresh token, it will not work and the attack will be noticed. If the attacker does not use the refresh token, it becomes invalid after the legitimate client uses it. This behavior is recommended by draft-ietf-oauth-security-topics

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.