Add support and testing for Worker MPM

```
mod_pagespeed has, as of this report, only been tested using the pre-fork MPM.  
There have been many reports about issues with the Worker MPM.

This bug tracks issues reported by users using the Worker MPM, where there is 
some reason to believe that the symptoms are related to the MPM.

```

Original issue reported on code.google.com by `jmara...@google.com` on 15 Dec 2010 at 2:20
