reloading: backport reloading fix

Issue #364 was fixed upstream in [r3492](http://code.google.com/p/modpagespeed/source/detail?r=3492).  Backport the fix to master.

"We only need to shut down the caches in child processes because they are not
started in the master process.  This wasn't a problem before, but shutting down
the caches can start a thread which isn't safe to do in the master process of a
forking server like Nginx."

Also backport the change from [r3497](http://code.google.com/p/modpagespeed/source/detail?r=3497) where we use a DCHECK for
may_start_threads_.  It's bad to start threads in the root process, but it's not
worth crashing a production server over.