Hi,
The leaked rewrite_drivers message occurs because mod_pagespeed optimizes resources in the background in response to HTML requests. After the HTML request is completed, there can still be optimization work going on. If Apache decides to shut down the child process, then mod_pagespeed attempts to delay the shutdown for about 1 second before giving up and shutting down anyway. Usually any individual optimization will complete within that deadline; even a complex image optimization. However if for some reason it takes longer then we may 'leak' the drivers and this is just noting that.
It probably should not be reported as an 'error'.
I'm not sure about the zombie processes -- it's possible that it's related but I wonder if it's also possible that it's another module that uses subprocesses, such as CGI scripts.
We have been discussing trying to tighten up the shutdown flow. One thing we could try to do is forcibly kill the threads that are doing the background optimizations, rather than letting them terminate cleanly.
-Josh