mod_http_oauth2: Allow requesting a subset of scopes on token refresh This enables clients to request access tokens with fewer permissions than the grant they were given, reducing impact of token leak. Clients could e.g. request access tokens with some privileges and immediately revoke them after use, or other strategies.

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.